This is a map made by Hellfighter.
It is a spoiler. Look at it if you are really stuck.
Citadelle 3

You have to go the room n°11 where there is a teleporter.
The lever to open the door is n°10.

You will find the key for the Battlefields 2 later in the game.

[Linked Image from squirrel.ovh]
duplouy.net badsquirrel.ovh
bad-squirrell.deviantart.com