Game over at 1am
The game ends abruptly at 1am (see 35442), even if Willy has collected all the items, or has reached his bed and is already on his way to the toilet, or already has his head down the toilet.
This fact seems to contradict the message that scrolls across the lower part of the title screen (see 33876), which implies that Willy must collect all the items before midnight.
The tune played on the title screen is Beethoven's Moonlight Sonata. The tune played during gameplay is If I Were a Rich Man from the musical Fiddler on the Roof.
The moving bed
The bed in Master Bedroom is actually a conveyor moving left to right, but it doesn't appear to move because the first and third bytes of the conveyor tile (shown below) are 85 (01010101), which remains 85 after being rotated left or right twice.
The music of life
When Willy loses a life, the pitch of the notes in the in-game music decreases, and their length increases; see 35668.
To prevent this from happening:
POKE 35674,126
Taking a BREAK
The game quits if BREAK - that is, CAPS SHIFT and SPACE - is pressed (see 35505). This works at any time, including when Willy is running to the toilet or already has his head down it.
If you take Willy to the floor at the bottom of the staircase in First Landing and type the word WRITETYPER, a cheat mode is activated that enables Willy to teleport to any room (see 35743).
For example, to teleport to Master Bedroom, hold down keys '1', '2' and '6', and then press '9' to activate the teleporter. For a list of all the rooms and their teleport codes, see the Rooms page.
As you were
Willy's animation frame at 34258 and direction flag at 34256 are not initialised before the game starts, so his animation frame and the direction he's facing at the start of a game will be whatever they were when he died in the previous game. (The first time the game starts, Willy's animation frame is 0 and he's facing right.)
White-seeking missile
The routine at 37310, when drawing an arrow, kills Willy if the arrow hits anything with white INK; this means, for example, that Willy would be killed if the arrow hits a white rope. It is no coincidence, then, that in the rooms that have both a rope and an arrow (We must perform a Quirkafleeg, On the Roof and The Beach), the rope is not white.
To see Willy die when the arrow hits the rope in The Beach:
POKE 34795,58 (start in The Beach)
POKE 64160,15 (set the INK of the room's background tile to white)
POKE 34790,128 (set Willy's initial pixel y-coordinate to 64)
POKE 34800,2: POKE 34801,93 (set Willy's initial attribute buffer location)
Beer in the wall?
The wall tile in West Wing is very similar to the beer mug item in The Nightmare Room; they differ only in the bottom row of pixels:
Dangerous connections
There are several places in Willy's mansion where it is obvious that he is in danger of falling from a great height and entering an infinite death loop. However, there are also some places where it is far from obvious that merely moving from one room to another will lead to an infinite death loop.
For example, the room below The Security Guard is Entrance to Hades, from which there is no escape.
For another example, the room above Dr Jones will never believe this is We must perform a Quirkafleeg, where the long row of nasties at the bottom of the pit ensures certain (repeated) death.
Unused room
There are 61 room definitions occupying pages 192-252, but the definition for room 47 in page 239 is not used. Room 47 is completely empty and can only be accessed by activating cheat mode and using the teleport code '123469'.
Unused grid location
The routine at 34499, which is responsible for randomly selecting one of the 180 grid locations on the code sheet by its index (0-179), never selects index 179 (R9); as a result, the code at grid location R9 is never asked for, even though it's present on the code sheet (it's 2423).
To give the code at grid location R9 a chance of being asked for:
POKE 34556,180
Unused items
There are five rooms in which nontrivial item graphics are defined, but not used.
Unused nasties
There are seven rooms in which nontrivial nasty graphics are defined, but not used.
Unused floor
There is one room in which a nontrivial floor graphic is defined, but not used.
Unused ramp
There is one room in which a nontrivial ramp graphic is defined, but not used.
Though unused, the ramp has its location set to (14,7); perhaps it was used at some point during development.
Unused conveyor
There is one room in which a nontrivial conveyor graphic is defined, but not used.
Unused guardian
The guardian whose graphic data lives at 45312 is unused.
guardian177-0 guardian177-1 guardian177-2 guardian177-3
However, this guardian made an appearance later on in the room 'Macaroni Ted' in Jet Set Willy II.
Unique guardians
There are seven rooms that contain a unique guardian (that is, a guardian that does not appear in any other room).
Commonest guardian
The commonest guardian is the bird, whose graphic data lives at 48128.
guardian188-0-66 guardian188-1-66 guardian188-2-66 guardian188-3-66 guardian188-4-66 guardian188-5-66 guardian188-6-66 guardian188-7-66
There are 23 instances of this guardian spread over 18 rooms: four in The Drive, two in each of The Bridge and Inside the MegaTrunk, and one in each of The Hall, Tree Top, Out on a limb, Rescue Esmerelda, I'm sure I've seen this before.., We must perform a Quirkafleeg, Ballroom East, To the Kitchens Main Stairway, A bit of tree, Priests' Hole, Under the Drive, Nomen Luni, Watch Tower, West Wing and West Wing Roof.
Commonest item
The commonest item is the bell, which appears in Up on the Battlements, We must perform a Quirkafleeg and I'm sure I've seen this before...
Every other item is unique to its own room.
Commonest codes
The commonest codes on the code sheet are 2411 (at H7, O7, R7 and C9) and 3443 (at O3, D6, L8 and R8).
Codes from code
The codes for grid locations A0-Q9 are derived not from a deliberately crafted data table but from the unused code remnants at 40448.
The one-way saw
The saw guardian in Cuckoo's Nest always has its blade facing left, even when it's moving left to right (see 41848). The reason for this is that its base sprite is 0 and its animation frame mask is 011, which means it cycles through sprites 0, 1, 2 and 3 only (see 37393).
guardian181-0-3 guardian181-1-3 guardian181-2-3 guardian181-3-3
All the other saw guardians in the game have the blade facing right when moving in that direction, because their animation frame mask is 111 (see 41376, 41384 and 41664).
Some air left?
The message at 33873 ('AIR') may be unused, but it is still a poignant reminder of Willy's former career as a miner.
Maria's dodgy depth perception
Whether Maria raises her arm at Willy in Master Bedroom depends on his height above floor level, as opposed to his distance away from her (see the code at 38224); this means that she raises her arm not only as Willy walks up the ramp towards her, but also if he jumps in the air at the entrance to the room.
Rope climbing for beginners
In order to climb down a rope, Willy must move in the same direction as the rope is swinging (left if the rope is swinging right to left, right if the rope is swinging left to right); to climb up a rope, he must move in the opposite direction (see 37756).
The encroaching rope
In the entity buffer at 33024, the definition for a rope uses the second and fourth bytes of the following definition (in addition to the eight bytes of its own slot); this means that to avoid corrupting another entity, the rope must be either the last entity specified in a room (in which case it will use the first and third bytes in the otherwise unused area at 33089), or followed by an arrow (which doesn't use the second and fourth bytes of its slot).
Ropes before arrows
The rope-drawing code at 37540 places Willy on the rope if it is touching anything else (Willy or otherwise) that's already been drawn. This means, for example, that if an arrow is drawn before the rope (by appearing before it in the room's entity specifications), Willy will be immediately transported onto the rope when the arrow hits it.
To see this happen in The Beach:
POKE 34795,58 (start in The Beach)
POKE 64240,60: POKE 64241,84 (entity specification for the arrow)
POKE 64242,1: POKE 64243,14 (entity specification for the rope)
POKE 34790,128 (set Willy's initial pixel y-coordinate to 64)
POKE 34800,2: POKE 34801,93 (set Willy's initial attribute buffer location)
The quadridirectional guardian
The cyan guardian in On a Branch Over the Drive moves up and down (not very gracefully), while its red and green cousins in To the Kitchens    Main Stairway and The Bathroom move left and right.
guardian185-0-13 guardian185-1-13 guardian185-2-13 guardian185-3-13
It is the only type of guardian that can be seen moving vertically in one room and horizontally in another.
Double descent
There is a maximum distance that Willy can fall before the landing will kill him, but it's possible for Willy to fall a greater distance than that if his descent is punctuated by a transition from one room into the room below; if each portion of his descent on either side of the transition is less than the maximum distance, he will land safely.
To demonstrate this, jump left near the bottom of the ramp in Conservatory Roof - Willy will land safely on the ramp in the room below (Orangery). But if Willy jumps left off the ramp in Orangery, he will die when he lands on a lower part of the ramp.
The code responsible for this is at 38115 - it resets the airborne status indicator at 34257 to 2 if it's currently 10 or less.
High jump
At certain points on a ramp, Willy can jump higher than the normal 20 pixels above his starting point. Depending on where he is standing on the ramp, he will jump 20, 22, 24 or 26 pixels.
When the jump key is pressed while Willy is standing on a ramp, he is drawn at the pixel y-coordinate Y, where Y is his original pixel y-coordinate rounded down to the nearest multiple of 8 (see 38344, which ignores the ramp below Willy when the airborne status indicator at 34257 is 1, and enters 38455 with B=0). So before the jumping animation even starts, Willy is already up to 6 pixels above the ramp itself.
Also, during a jump Willy rises and then falls until his pixel y-coordinate is Y again, and if the jump key is still being pressed, he will jump from that point instead of falling all the way back to the ramp first.
Guardian colours
The most common guardian colour is yellow (25 instances), and the least common colour is blue (2 instances, both BRIGHT, in The Forgotten Abbey and The Attic).
As for the other colours, there are 18 red guardians, 20 magenta guardians, 19 green guardians, 18 cyan guardians, and 7 white guardians; there are no black guardians anywhere.
Guardians need a clear path
The guardian-drawing code at 37334 kills Willy if a guardian collides with anything that's already been drawn in the room. This means, for example, that Willy is killed if a guardian hits a nasty, a wall or the floor.
See this happen by placing a wall block in the path of the guardian in The Bathroom:
POKE 57635,128
In addition, if an arrow appears before a guardian in a room's list of entity specifications, Willy will be killed if the arrow hits the guardian. See this happen by replacing the first guardian in Top Landing with an arrow:
POKE 58096,60
POKE 58097,56
Guardian or fixture?
Entity 89, defined at 41672, and appearing in The Hall and West Wing, is the only guardian that does not move up, down, left or right.
Unanimated guardians
There are a few vertical guardians that have only one animation frame, and so are in a sense unanimated (though they do still move up and down).
In contrast, every horizontal guardian has either 4 or 8 animation frames.
Unused entity definition
Entity definition 43 at 41304 contains data but is not used.
Which room might this vertical guardian have inhabited at some point while the game was being developed? We'll probably never know.
Untouchable entity definition
Entity definition 127 at 41976 should not be modified or used to define an actual entity, because its first byte (255) is used to terminate the entity buffer at 33024.
One item behind another
There are two items in The Beach, but they are both at the same location, so it looks as if there is only one.
To move one of the items two cells to the left:
POKE 42438,180
Items and whiteness
If an item lies in the path of an arrow or white guardian, it will be collected when the arrow or guardian hits it. This is because the routine at 37841 collects an item if the cell it occupies has white INK.
To replace the guardian in The Bathroom with an arrow and watch it collect the item (make sure Willy jumps as the arrow approaches):
POKE 57840,60
POKE 57841,214
To make the guardian in The Bathroom white and place the item in its path:
POKE 41433,103
POKE 42239,33
POKE 42495,135
In addition, any item situated in a room that has a background with white INK will be automatically collected as soon as Willy enters the room (see The self-collecting item).
Slippery slopes
In The Chapel and Halfway up the East Wall, the ramps act as conveyors that push Willy down the slope, because the ramp tile has the same attribute byte as the conveyor tile (which is not actually used in either room). There are no conveyor-ramps anywhere else in the game.
To pause or not to pause
If the in-game music has been switched off, the game will pause automatically after a period of inactivity; however, if the in-game music is playing, it will not (see 35644).
Cheats never cycle colours
While the game is paused, the INK and PAPER colours are cycled at regular intervals, unless the WRITETYPER cheat mode has been activated (see 35553).
Nomen Luni?
The name of the room Nomen Luni is a pun on 'Nomen Ludi', a Latin translation of 'The Name of the Game', which appeared in adverts for the aeroplane shoot-em-up game Zzoom by Imagine Software Ltd.
In Nomen Luni and the room below (Under the Roof) you can see an aeroplane that has crashed into the roof of Willy's mansion:
We must perform a Quirkafleeg?
The name of the room We must perform a Quirkafleeg is a reference to issue 5 of the comic book The Adventures of Fat Freddy's Cat, in which the inhabitants of Pootweet, upon seeing a suitcase full of dead mice, exclaim 'We must perform a Quirkafleeg!' and promptly lie down on their backs and start waving their arms and legs in the air.
Rescue Esmerelda?
The sequence of rooms Up on the Battlements, We must perform a Quirkafleeg, I'm sure I've seen this before.. and Rescue Esmerelda - with the guards, battlements, rope, bells and Esmerelda - are a reference to Hunchback by Ocean Software Ltd.
Dr Jones will never believe this?
The name of the room Dr Jones will never believe this may be a reference to the 'Myth of Jones', a story told by the philosopher Wilfrid Sellars in his most famous paper, Empiricism and the Philosophy of Mind. In this story, the hero, Jones, seeks to explain how a person can have a sensation of something that doesn't exist; for example, how it can be that people behave just as they would if there were a pink elephant in the room when in fact there is no such thing there. In this particular room, however, there is a pink (well, magenta) elephant.
The emptiest rooms
Back Door is the only room that has no items, nasties, rope, arrows or guardians.
The Front Door is the only other room that has no rope, arrows or guardians; it also has no nasties, but does contain an item.
The most guardians
The Forgotten Abbey is the only room with a full complement of eight guardians.
The Attic is the only other room with eight entities defined: six vertical guardians and two arrows.
The most items
The room with the most items is The Off Licence with 12.