Three doors, three entry points, one test. Each subtracts a character's first sprite from the player's current one and asks whether what is left is under $10 -- which is exactly "is the player this character", since each character owns sixteen consecutive sprite codes. DOOR_SERF takes $21 for the serf, DOOR_WIZARD takes $11 for the wizard and DOOR_KNIGHT takes 1 for the knight. The records that reach them are types $1A (a barrel), $17 (a bookcase) and $10 (a grandfather clock), drawn as graphics $BB, $B8 and $B1 -- handler-table entries $BC, $B9 and $B2.

Pass the test and the thing behaves as an ordinary door, through the same DOOR that every other door goes through. Fail it and control goes to DRAW_DOOR instead, which draws it and nothing more: the door is there, visible, and will not open.