The lich defense is very strong, but it is very difficult at the beginning of the game and in the middle levels have a decent defense. In the last levels you are invincible because you only investigate spiders and scorpions. Personally, I recommend that you save your diamonds to build gold snipers and potions ... and when you reach level 39, build priests ... The weakest defense in my opinion is the human and the dwarves.
The dwarves, it's like lich. Until they reach level 39 and build golden level 13 mechs, they are very vulnerable to an attack composition with swordsmen and rockets.
The human defense is very good in the initial phases of the game, but later in the advanced levels they die very fast.