Nice game, like the concept! I like there's a variety of units, and two pools of resources (nice yin-yang orb by the way :D) I also like the resource management aspect of it. Found the art and sound fitting. Extended options menus - you can even rebind controls!
I was focusing on fast units at first. But that easily left me vulnerable when a monster appeared on one side and everyone rushed there, and another one came from the other side and reached the castle. Then I tried focusing on slow units and spreading them around the castle. Eventually they also converged on one side, leaving me vulnerable on the other. Maybe some of the units should remember where they were spawned and try to stay in that area? Or perhaps don't make everyone rush over for a single enemy. Or maybe have the defenders have an aggro range so they don't rush from one end to the other, and make them return to the castle when idle.
It might be a bit harder to defend the top and bottom as there's less screen space there :D
Finally, at times it was hard to distinguish my dude among all the other units. I'd make it a bit more visible (or add some outline / highlight). At times the enemies were a bit hard to spot too.