[Wyatt Hebert]

I have noticed that Spies, at least, _do_ let you know what's in a Castle you are sieging. Also, I'm not sure if someone explicitly stated this, so I will:

In my experience, very rarely do computers defend their provinces with Castles. Your attack will kill their PD, but not any regular army in the province. Any units in the province will be inside the castle which you are sieging.

For your case, just do the numbers, and you'll see why you can't hurt the walls.

1400 units vs your ~800 or so castle damage. Not nearly enough. otoh, they won't be able to feed 1400 units nearly long enough. If you can handle that army sallying out to meet you, you can just wait them out. Or, as people have said, surround the place with temples, move in a bunch of priests, and preach them out!

Wyatt