I really love the game and would add:
And I would suggest few things:
Add a Hero- A main character that grows. A bit more personal than the castle.
Add a storm element to destroy the flying ships
Add armour spell - Prevents damage for a time
Add enviroment damage, dropping ships makes damage - burning trees kill
Add water, there could be a flood
Add change environment - Making a hole would be interesting and working strategy
Add static tower. A sniper or a arrow shooter with constant pay - 1 point in 10 secs or so..
it is endless. Ill be watching the updates closely :)