Center pulls out the portal key, but shakes his head. What he has is a a black metal triangle, three inches across, with an inverted "Y" inlaid in copper. Unlike the portal key taken from the lich, it does not have an actual key (of the sort which can be inserted into a lock and turned) attached to it.
As noted above, with Mindsight you detect a great many aberrations of animal intelligence (2) beyond the doorway. It must be some kind of swarm.
The door has not been opened, so it blocks the spells which rely on sight. There's no magic on the door itself.
Your search of the door does not reveal any traps.
You think you hear some sort of motion from beyond the door, but you can't tell much else.