Okay, I read about the aims of the game and solved the first 2 levels. But now in level 3 after I killed all worms and birds, my ballon just sits above a building and doesn't collect the mana. So I try to build more/better buildings/a castle but when shooting the builder stone, nothing happens. No matter if I shoot at any building in that level or just anywhere on the ground. What am i missing?

2nd post attempt:

I had a similar problem after placing my fort to close to an AI one. Destroying the AI fort's outer walls get me the space to upgrade. I've only had this happen with AI forts but perhaps there are other things that can get in the way.

I didn't play this back in the day so I'm a newb.

I used to play this game a lot and I think I remember some landscape elements are indestructable, building to close to these means you can't expand your castle, you have to demolish it and build it elsewhere. Think you can do that by using "shift L"