Here is the full list of all 12 Royal Defense trophies - there are 7 bronze, 4 silver and 1 gold.
Build 50 towers
Kill 1000 enemies
Kill 3 enemies with one spell
Build 50 mines
Sell 50 towers
Kill 500 enemies with spells
Make a complete improvement for 50 towers on the levels
Fully upgrade any tower in the store
Build 1000 towers
Sell 500 towers
Kill 30,000 enemies
Complete all levels to 3 stars