I think I agree for the most part with Ama on this one. However, as far as there not being any "booby trap" for flying troops, I think that's where the upgradeable towers could come in. I like the idea of being able to garrison troops in a limited number of towers. I played Evony, did very well in the game, and part of what I didn't like about the game was that a lot of it wasn't really all that realistic. Tens of thousands of archer towers on that little wall?!? Lol... come on! However, it was a base defense that regardless of what direction you attacked from, realistically, much of them WOULD have been "triggered". Garrisoning would be cool, and as far as flying units, maybe these towers could be upgraded with ballistas? Like if you could purchase up to 10 of these towers max, maybe garrison a maximum of 100 Longbowmen in each one, and it would provide 5x damage? So completely maxed out, someone could garrison 1000 archers in their walls and it would be as effective as 5000 in a normal battle. It would just add a bonus to the defender. Then if you wanted to have a small bonus against air units, you could build a ballista on each tower for a high cost. Keep in mind that this bonus would only be applied to 1000 Longbowmen. Just throwing it out there! Fun game so far, day 2 for me :-)
Best argument for wall defenses I have seen yet.