It was a bit confusing to me on my first try how the buildings work :D Maybe being able to access the instructions from within the game could have been good, or maybe a tooltip to remind you what each building does.
I think even if you don't have any troops (from clicking the barracks) and then click on one of the walls, the little blue squares still spawn, which threw me off a bit. So on my first run I didn't really understand how you were meant to defend.
Clicking so much did get a bit tiring :D Would be cool to have some passive troop generation.