Into the Eagle's Nest

01 · The floors

The castle, floor by floor

Each floor as the game builds it when the player arrives, drawn with the play screen's own tiles and colours. Point at a tile to see what it is.

A floor's description is 32 rows of 32 bytes, kept in the memory under the input and output chips at $D000, $D400, $D800 and $DC00. Each byte names one of the blocks at $7000, sixteen bytes each, four tiles across and four down. When the player arrives on a floor, the builder at $6A4B copies every block into a map 128 tiles each way at $9000, so a kilobyte of description becomes sixteen kilobytes of map. The screen shows 28 tiles by 21 of it at a time.

The maps here are built the same way, and a test runs the game's own builder on the kit's 6502 for all four floors and checks that the page builds the same map, tile for tile (test_maps.js). In a snapshot of the first room of a mission, the game's map differs from the page's in 171 of its 16,384 tiles: sixteen are the player, stamped into the map, and the others moved with the guards after the map was built.

02 · What a floor holds

What the builder picks out of each floor

Every byte of a description is a block, and a few of them are something more as well.

A byte from $30 to $37 is a guard. It names a block like any other, which has the guard drawn in it, and the builder also puts the guard on the list of figures at $8100 to $8500, with its place in the map and the direction it starts out walking, two description bytes to a direction: $34 and $35 up, $30 and $31 down, $36 and $37 left, $32 and $33 right. From then on each guard turns towards the player as it walks ($630B), and up to seven of them take a step each time the figures have their turn, on two frames in three.

$62 marks the floor's dynamite charge, whose place the builder keeps in $02EA, and $63 the prisoner, kept in $02ED. The charge stays on the map only on the demolition mission, and only until it has been armed: on every other mission, and on that one afterwards, $5F9E clears its four tiles as soon as the floor is built.

No byte of a description means a chest. Two of the blocks, $3F and $43, have a chest drawn into them, so a floor has a chest wherever its description names one of those two blocks, and what the chest holds is decided only when it is shot (Discoveries).