Fun game! I tried the dwarves and the merfolk, and they both had some interesting abilities. I had some problems with getting the siren to create water where I wanted - when I clicked a spot with an enemy in she went up and punched it instead.
The AI is pretty bad at playing, but that's understandable, it's hard to design a smart AI. Maybe include an option to give the two sides different amounts of gold? That could be useful in multiplayer too, if you want to give one player a handicap.
It will be interesting to see where this game goes.