Spenser was not impressed. His fur was standing up, his tail was getting puffier and puffier, and he'd progressed to full-on yodeling.
The other cat was unfazed.
By this time, the remaining three cats in the house had come into the room, to see what was going on and ensure they weren't left out. Feargal had started to mew, too.
The other cat still wasn't going anywhere.
Then Spenser launched himself at the window. He did not, fortunately, go through the screen, but he gave it his level best. This finally convinced the other cat that it might be more interesting to go elsewhere for a bit, so he took off. Spenser sat at the window and seethed.
I then spent a while petting him and praising him, and telling him what a brave boy he was, defending the house like that.
I'm really glad he didn't go through the window. Given what happened last time he was exposed to the great outdoors (he felt grass for the first time and freaked out), I don't think it would go over well.