Defend your castle

Well, if I remember correctly, you get a bomb or something after a few levels from the demolotion lab. I would suggest getting a mana pool though, the wizards are really useful when the giants come.

The demolition lab turns your men into walking bombs. Click on a guy you have converted and he runs out and click on him again to explode. Workshop trains builders, which repair your castle during each level. A mana pool is definitely worth it if you can get enough upkeep to not kill off all your units at the end of each level. If you have a lot of wizards and archers, and can keep them, then you are set.

if you get about 500 archers youll find you dont need anything else, and that it becomes hard to convert more people since they die so quickly

One more thing, my units died after one of the levels. Is this preventable?

Yeah, they die when your upkeep exceeds the number of points you have available to spend. Basically earlier on you have to be cautious with the number of people you train. Once the points start flowing in it won't really be a problem, but for the first few levels it's important to keep some in reserve instead of spending them right away.