I really like Rook, very nice to see and to play.
But the equipement does not really fit, you don't put a ring or a armor on a big stone giant !!!
A idea could be :
Instead of buying equipement, we should buy castle upgrade !
Remove the arrow tower / trebuchet tower / light tower from the skill tree, and add it in the equipement store. And for each slot, we should choose between each upgrade. It could be a very rich feature. Some examples :
- I add trebuchets everywhere, to have a strong, long range, offensive power against building
- I add light towers everywhere, to be very good in my territory (good defense)
- I add arrow tower everywhere, because I love to send rain of death on my enemies
etc....
I think the idea is very good, it is not really from me : A similar mechanism could be find Universe at war. Good idea should be copied ! 
If you keep castle upgrade in the ability tree, let us choose for each shoulder/head the tower we want.