Although I think in general this game is well designed, there are a couple of things that really bug me.
The first is the clumsy method of adding new units. You need to have an empty slot in your castle's defenders before you can add a newly built unit. So you have to move a unit to the inventory before adding the new unit. I got so sick of doing this that I now have one permanently empty slot in my castle's defenders so I can add a new unit right away. I think a new unit should go straight to the inventory if all the defender's slots are full.
The other complaint is more substantial. It appears that only the units currently defending your castle are listed when you attack another castle. So if you want to use different units for attacking than defending, you may find yourself swapping units in and out of your castle's defenders when you want to attack or go back on the defence. The other disadvantage is that you don't have your full range of units to choose from when conducting an attack, and since you can't see the enemy castle before attacking, you don't know what the best mix of units will be for that attack. These issues could both be resolved if ALL your units, whether in the field or in the inventory, were made available when you are conducting an attack. It would also add to your possible strategies when attacking and so make the game more enjoyable.
Oh, one other thing. I really don't need a couple of big windows pointing out to me where an item went in my inventory when I buy something in the shop.