An excellent game
A few things did bother me, which is why I'm giving a 9 rather than 10.
I liked the graphics and music, as well as the idea - guard towers which spawn units aren't common in these games!
* It's great that it has a story, unfortunately the story here often gets confused due to the non-linear selection of battles (a good feature in itself). It is obvious in some points that I was intended to go through the cities in a certain order, and I didn't. I don't have suggestion to solve this though... :)
* A small related bug - I could see descriptions of monsters in the Great Library after the city in which they appear was accessible, but before I actually accessed it.
* Sometimes available guard posts are overlaid with the scrolling arrows and are difficult to select.
* As mentioned, a way to bring in the next wave would've been nice (pausing is possible by entering the menu).
* The random attacks on my cities made things a bit more realistic, but the fact that my only option is whether to spend resource or not, to improve unknown chances of success by an unknown factor, was kinda frustrating.
* I replayed maps mainly to get the best ranking, earning resources on the side. Near the end I felt I might need to replay the easy maps for resources, which would have been quite boring. Might have been nice to get a larger resource bonus for [re]playing more difficult maps.
* There was no convenient way to show the tutorial instructions again - they weren't displayed when I replayed the tutorial map.
Still, these are just small bugs in an overall great game.