Lets see, think of how many levels you want.
Pick a type of unit for each level.
Make their armor and health increase with each level.
make a path or some that they are required to run along.
Place triggers to create each unit accorind to level, and to make them run.
Make teams, and tower for each team. perferable make tower with different attributes.
Maybe set bounty for each kill, or level.
I think that is about it. Test it so you know it's beat able