Yea the "Defeat the castle at the end" objective isn't really clear, I didn't have too much time to finish it up and I forgot to add an attack animation and health bar to the castle soo... yeah sorry about that :P
I'm also quite bad at making tutorials and sadly, this is the best one a made so far :'D
If this gets made into a full game, I'll make some tutorial mission, that should be good for an explaination.
Also, thank you for taking your time trying to understand the game, we really appreciate your effort :D