Hello !
I have just tried that variation of a tower defence : a tower conquest (you are the bad guy sending troops). http://www.kongregate.com/games/Rete/villainous
And somehow, I can't help thinking that there is a link with desktop dungeons (maybe the evolving castle, the fact that you keep coming back to the same dungeons with different competences, or that even if you have lost a siege you still managed to gather some wealth). It could maybe fuel some ideas...