Tower tactics.

Personally, I believe that the most important thing when playing as Tower is to constantly upgrade your knowledge skills. I'm not even talking about the main hero; he needs to be leveled up, but if he's far away and a strong enemy is approaching your capital, you can defend the city even with a basic unit if it has at least 4 points in knowledge. And, of course, have upgraded Djinns and/or archers (golems, naturally) in the garrison.