[Kuritza]

With tarts, you can hope to break siege and try to re-conquer some land. With gemgens, you can keep summoning, but you are still losing because your opponent has BOTH gemgens and the land (hence money and recruitables). Am I missing something?

Just bring some siege golems to breach the walls.
When it comes to unbreakable castles, Pangaea is much more annoying (to the point of being unfair, in my opinion). Pangaean Pans are so gaean.