1. Before the first wave, you need to buy as many archers and ~3-4 swordsmen as possible. Place archers on towers, the more towers the better. Survive the first wave.

2. Now you need to use all your forces to destroy the nest and the cave with the undead at the bottom of the map. After that, build a sawmill and a quarry. A mill would not hurt either. Buy even more troops. Wait for the second wave.

3. During the second wave, you need to lure a zombie (preferably one) down so that it starts breaking your buildings. This can be done with one swordsman by placing him in the passage. Do not kill the zombies under any circumstances.

4. After you kill all the main force of the second wave, you will have one zombie left at the bottom of the map, which is easy to lead back and forth with the help of a swordsman. The third wave will not start while he is alive. Accumulate resources as much as you want. Develop the settlement. The zombie will wander from the sawmill to the quarry and back, breaking everything along the way. Do not forget to build wooden towers on his way so that he has something to do. Do not kill the zombie. Keep your swordsman aside in case the zombie wants to go to your main settlement then redirect him down.

Almost forgot to say. Don't destroy the last cave until the very end, otherwise you will break the game. The waves will stop and you will have to start over.

Good luck.