This game is awesome!
I made a basic guide for people that wants to know more info about the buildings.
Ninja Kingdom Guide
I personally like the following units:
Assassin - Your main damage dealer. Keep them safe from the demon mortar.
Samurai - Your tanky front line to soak up damage. Use them to protect your assassins.
Ninja - Use these to quickly take out towers.
I tend to mix these 3 together.
60% assassins, 10 % Samurai and 30 % Ninja's + Oni always works well for me in raids.
Good luck guys!