I seemed to be doing quite well until windows rebooted my machine during my lunch.

I had, essentially, duplicated incarnate's achievements above but my structure and shields were still 100% intact and I had gotten it down to a pretty good science about how to defend my structure.
I think "recharge shields" might be a little overpowered. During a large battle, I would task my wizards with generating mana, place the apprentices on repairing shields, imps gathering gold, golems gathering stone. And I'd use my mana to recharge my shields. I had enough attacking power that I would, generally, eliminate more of the attacking force than I lost each round which allowed me to escape with only minimal structure damage.
Since I chose the living tower option, my structural damage would be fully regenerated before the next major battle.
But, I have to say that this game is VERY fun!