Good stuff!
To all those people who complain, the first time you play is very hard, yes, but then you get achievements, more money, etc... and it becomes a whole lot easier. Seriously, WAY easier. Just stick to it. Also pay attention to what Nation you are playing as and against. Hint to the engineer haters, Prussia has weak engineers, France as great engineers. But Prussia has grenadiers, those guys can tear up your walls if you let 'em. Try starting as Britain vs Britain.
Ideas. Well, I never used traps. Traps were a nice idea but they were not worth it. Caltrops aren't very effective and mines were too expensive. It was easier just to throw up another wall or save for better troops.
While it probably would make the game much harder, look into cannons for the attacking force. Nobody threw bodies against a stone wall back in the day. ;)
And last, once you get the right achievements the game got very very easy. I beat the hardest level loosing only one guy on the last wave when I had wooden walls. Perhaps making units gradually get stronger on the hard levels? Just an idea.
I had a great time playing, SpaceCat. Thanks for the awesome game. It ate a good chunk of my day and I hope to see more games from you. Ignore the naysayers, this was a very well rounded game and I liked it a lot, that's why I went into the little nitty gritty stuff. Once again, just to drive the point home, Awesome game!