I found two entrances: A melon farm had a cave in the roof that led to a massive sprawling series of more caves, some of them unclaimed, unexplored and/or likely connected to the surface.
At the bottom of the steps, there was a hole in the roof that allowed entry-only bypass of a door, plus it led to mining tunnels in unclaimed land.
The rail leading from the bottom of the stairs to the spider spawner connected at multiple points to various tunnels almost certainly some of them unclaimed.
I sealed the first two, and put a door before the second, so there should be fewer entrances.
Additionally, walking along most of the minecart passages allows you to get to many areas without having to pass through a door. This means that if someone *does* make it in, they'll have access to most open areas. This isn't much of an issue for treasures (They *are* stored in secure locations, right?), but it leaves most of the fort as a risky area to be, as you can easily be killed, reducing the total points, potentially to the point where some of our land can be claimed from us. Worse, if they're clever, they'll find land that has an essential door or two, so that they have even easier entrances to break in through. They can also claim some land and then sneakily construct back door tunnels that lead in from outside that bypass all of our doors, connecting them to points that are hidden enough that nobody will notice the new security holes.
I propose that _all_ rails and rail stations are isolated from the rest of the area by self-closing doors, and any access corridors to mechanisms have doors. Also, people should close doors behind themselves wherever possible. Also, people should think about putting sets of doors in the middle of passages, so that if someone does make it in, they'll only have a limited section to wander in.
Finally, can items dropped by opponents trigger wooden pressure plates? If not, what about chickens spawned by eggs thrown by them? If not, what about eggs dropped by chickens dropped by thrown eggs thrown by enemy factions? Better use stone wherever possible, for secure doors. Or, better yet, buttons. Since the Minecraft wiki says mobs trigger stone plates, and chickens can be spawned from eggs, which are portable.
<New replied while i was typing>
Wait, did all of that just happen?
Oh, another thing: I reccomend putting the spawn point itself in an inacessable exit-only room.