I like it, but there could be a few improvements.
The biggest thing for me is that, if you enable mouse scroll, clicking on the spell icons can be annoying because you start scrolling left whether you want to or not. I like spells, and i like mouse scroll. Is there some way these two can coexist (like putting the spells in the center of the screen)?
Also, I think the game would be a lot more interesting if there were increased diversity with the units so they're not just "melee," "better melee," "ranged," and so on. Upgrading the units via skill points is good but it takes so long to upgrade them that way (several levels for some) that not everyone is going to get the benefit. I'd suggest making the units more unique from the get-go. Maybe the slaves never get armor upgrades (they're slaves, who's giving them armor?) but they have the fastest movement speed? What if the goblin soldier had a chance to block attacks with his shield and the assassin had critical strike (to start)? Something like that. Another option would be to vary the army with the hero. Maybe the warlock's mages are stronger, or come with free slaves to serve as meat shields?
Again, I really like this game. I'd just like to see... more.