GOod start
A good start but I thought it had two flaws:
1)what happens to your surviving troops? I think that if they survive the battle you should get them for the next round. THis would also help when you make the game more difficult. It seemed a little easy and it being easy also made investing in unit upgrades pointless.
2) it grew a little boring, buy troops attack castle. what if you also had to attack enemy armies or villiages? then it would truly be an invastion game!
But other than those two faults GREAT GAME keep up the good work