It would be nice if mousing over a button would highlight it, so it is clear that you are selecting the right thing. Maybe have a short gap after each wave too, that way you have a little bit of time to prepare without pausing.
There is a definite lack of direction at some points, quest markers on the map would have helped. Side quests seemed pointless due to crappy rewards. The ending was kind of predictable and a let down, no boss battle? Just a bit of text and then a request to donate :/