[color:"orange"]Act 1[/color] (game version 1.47)
Citadel guard armour and key (14KB) - dropped by the first guard you encounter in the citadel.
Any citadel armour will work as a disguise, so you would also be able to use one bought or found in the BF. There are a couple ways to get through the door without a key (see below), but since you may have to go back for certain items (if you missed them), it would be better to use the key.
armory trick in act 1 (Spoilers)
Teleporting anywhere (via hex editor) 
