Very much enjoyed playing through this, although I think the progression could use some work. If you destroy the castle too quickly it becomes basically impossible to defeat the final wave. A good work around for this might be to give the player extra coins whenever they add a bone to the cauldron? Additionally, the only way I have been able to beat the game is with a summon + bard combo, anything else I've died at the final stages with. It'd be nice if some of the stat-focused units leveled up a bit more strongly. I think what would be most helpful is allowing you to upgrade a Level 2 unit with just one additional Level 1 unit, rather than requiring you to combine two Level 2 units. That would make it much more viable to use units that don't spawn other units.
Overall a great game though, would be interested in seeing something like this as a full game with more fleshed out skill trees, additional levels, etc.