Cool concept! Honestly with some tweaking it could have lots of potential. There are a lot of values and so balancing gets really important but there really is room for a lot of depth and strategy.
Also I just wish that the enemies came from all directions so it's more satisfying to siege the castle haha. Maybe instead of the tower upgrading, it instead places down turrets to defend or places factories to upgrade itself?
Overall really nice entry. Kudos!!