Wow. Just wow.

Now, one of my friends mentioned a game when he was in a situation like this. When lots of high-level adventurers are summoned by mysterious entities to a heavily fortified place, it's almost guaranteed what's going to happen next.

I think you're going to be besieged. Thousands of zombies, most likely, with spellcasters to break down the walls, and maybe some demons. I think you'll be wanting lots of area-destruction spells, as well as a few battlefield control spells. Lay down traps around the keep. Start looking for potential traitors among the other adventurers now, and hope that the orders of binder knights are not themselves traitors (that is, they lured you and the other adventurers there so they could eliminate everyone strong enough to be a threat to their new world order).

Granted, the kython attack on the village and the siege of the church may mean that the DM has filled up his siege quota for this campaign, but when a DM tells you a place friendly to you is heavily fortified, you're probably going to need to use the fortifications.

Good luck, and I await the next update anxiously.