This is a spin off from another thread ware I suggested a noble that would control the gate or door to your fort. After a bit of brainstorming I'm fleshing out the idea and looking for feedback.
First off the Gatekeeper has to be under the players nominal control for this to be a useful noble (their should really be a distinction between the useful 'bureaucrats' and the useless 'aristocracy', this guy will be a member of the former which people generally like). The player has to put a Doorway or bridge under the control of the Gate Keeper for anything to happen, this would just appear as a new option in the door menu as a kind of in-between lock and unlock.
Second creatures need to understand that doors can be opened for them, especially caravans and traders, wagons shouldn't bypass the fort if the drawbridge is up, rather then come to the edge of the bridge or door and 'present themselves' for admittance. Which in game mechanic speak means they telepathically transmit their desire to pass through the door to the GateKeeper, they then wait a reasonable time before trying another path or leaving in a huff. The GateKeeper will either accept or reject the request (more on this later) and if accepted an open door request will be initiated. Note that only Keeper controlled doors will licit this wait behavior, locked doors are just passed over as they are now.
Actually opening the door/bridge might require a lever pull in which case the player will need to indicate which lever is appropriate to pull at the time they set the bridge/door in question to Gate Keeper control. For a conventional door the GateKeeper dutifully comes over and opens it. Or if the player chooses on the door menu a member of the Fortress Guard may open the door, much like the Trade Depot allows someone other then the designated Trader to trade. Their might be some downside to this but I'm not sure what would be appropriate yet.
Lastly the ability of the Gate Keeper to reject some prospective entrants needs to be controlled on the Noble screen as this is THE key feature for a Gate Keeper. Theirs almost no end to the possibilities here, the simplest options would naturally be to accept or reject the various trade caravans, don't like elves (and don't want the bother of cleaning up the drowned bodies) just don't let them in, they should eventually get the idea and stop coming all together. If some of the ideas on random visitors are ever implemented theirs another thing you can keep out. Perhaps you can let in merchants only if their buying or if their selling or have a particular commodity. Migrants are another thing people want to have control over but theirs already an excellent thread discussing an officer specifically tasked with population management which is a very different issue so I'm loath to try to control immigration through the Gate Keeper.