Snapshot of the last 24 hours...
I was in Penwiper's Suite (AKA the library) and saw that we had spare X-mats, spiked mats to keep cats away from an area.
Penwiper had already tried exploring with a paw under the door, so I decided to discourage him by putting an X-Mat under the door when I left. Presto! No more paws under door.
Within seconds of my leaving, P.W. had his paws under door as he meowed, and then we saw the X-Mats being shoved out of the way by unseen forces.
We won't even talk about how good he is at escaping when we open the library door. Suffice to say on his latest escapade, he made it past all three senior felines and all the way down to the dining room before I caught him. Proof, fortunately, that they can mostly co-exist.
(It should be noted that Oscar is stressed by Penwiper. This morning in the hall, Oscar heard P.W. meowing in the library as Billy walked by, so he hissed at Billy. Clearly, that needs work.)