Decent graphics and animation.
It gets very repetitive after a time - you always have to defeat the same 'easy" waves before you can get to the more challenging ones, which is very boring. Maybe you could have added "checkpoints" to allow the player to start at a later point?
More critically, the "drag and drop" method of changing your formation in battle does not always work properly. Sometimes the circles demarcating your different groups don't properly follow the mouse cursor, and "get lost" off the edge of the screen. You have to click several times to release, then start over. This is NOT a good thing to have happen in a critical battle. Also, your groups don't always seem to drop in the position where they're supposed to be. Furthermore, many of the icons indicating your different units types look too similar to each other. For example, the Demon and underworld warriors.