Every room in the castle: 128 records of varying length packed end to end, the last ending where block_type_tbl begins. There is no index and no terminator. find_screen finds a room by walking the records from the start, comparing each room number with the player's; byte 1 of a record is the distance from itself to the next record, and the walk gives up when it reaches the address of block_type_tbl.

Byte 0 is the room number. The castle is a grid of 16 by 16 squares, and the number is the square's row times 16 plus its column: walking out through the east or west side adds or subtracts 1 in the low four bits only, and north or south adds or subtracts 16 (screen_west to screen_south). Only 128 of the 256 squares are rooms. Byte 1 is the record's length less one. Byte 2 holds the room's ink colour in bits 0 to 2 -- magenta, green, cyan or yellow -- which found_screen makes into bright ink on black paper at $5BAD, and its shape in bits 3 to 7, an index into room_size_tbl.

Next come the room's backgrounds, one byte each, indexes into background_type_tbl: walls, arches, doorways and the other fixed scenery. A byte of $FF ends them. A room with nothing more in it has no $FF and simply ends; 24 rooms are like that.

After the $FF come groups of objects. A group begins with a byte whose bits 3 to 7 are an index into block_type_tbl and whose bits 0 to 2 are the number of copies less one, and carries one position byte per copy. A position byte names a cell in the room's grid of 8 by 8 cells on four levels: bits 0 to 2 the x cell, bits 3 to 5 the y cell and bits 6 and 7 the level. next_fg_obj_sprite turns it into coordinates: x is $48 plus 16 times the x cell, y likewise, and z is the floor height plus 12 times the level, to which the block type can add half a cell in x or y and extra height.

retrieve_screen builds the room as object records, 32 bytes each, from $5C88 upwards, and clears whatever is left up to font. That is room for 36 objects, and six rooms in this table need exactly 36.