I think in this game zombies are too smart like how a rotten hunk of flesh have so much knowledge about when the horde is attacking the fort and that it's best for them to attack at the same time as the horde to take like 3 blocks from my fort. And it happens every time around 20 day turn.
On level 14, built CRYPT on each high ground close to the EXIT, upgrade FEARFUL CRYPT to the max. And keep all your mana to throw at the final cloaking boss... It's gonna be hard and kinda RNG if it freezes the boss then you gonna win, if it gets feared away from your games then don't miss any meteors, if anything else like it isn't feared or it's feared into your gems then it's like insane difficult to hit meteors all in time.
tip to beating level 12 : first you must buy barrack keep and windmill then build transport after that send out 3 footman then after they all die send 1-2 more footman after that there will be left 1-2 enemy units that attacking your base, let them stay alive, becose enemy AI is an idiot and won't send any more units. Then try to asap go for economy and best units in game, becose if you don't then you'll never get to enemy base of enemy unit span rate. After maxing out try to spawn 6-7 high priest, 7-6 cannons and 5-4 or more generals to win level 12. Its impossible to beat this level without taking any damage on your base (beating it under 8 min is omega hard or impossible).