Crossing this with tower defense was a really fun idea!
My first couple of plays, I didn't get very far and couldn't figure out what I was doing wrong. I was summoning pawns and trying to advance them across the board right away. On my third play, I figured out that I needed to keep forces concentrated closer to the center in order to better defend my tower, and I made it a lot further--until the first creepy black hand-thing showed up, I think the third type of enemy.
My biggest suggestion would be to add a free-scroll control to the game, and maybe the ability to zoom out. Having to click on units to navigate meant that it was hard to get the fully lay of the land, and I could even lose a knight far away in the edge of the board if I wasn't careful. The area of play doesn't seem too big.