Here are some more ideas, if you want a specific trigger code, you must, ummm, specify it.
Bosses
Final Boss
Immune
flying
invisible
walking
maybe levels that stun towers?
levels with auras that remove part of the dmg of the towers
creates, like a Phoenix, after they die, they turn into an egg, after that again into a phonenix, so you have to kill the egg
levels that make more creatures or duplicate themselfs
super fast levels
Commands such as -air(displays air levels), -immune, -boss and so on
Sell
kick vote trigger
Heroes
Units - not recommended
if a player leaves give all towers randomly to the players and remove the spawning for that player side
Different Races
Some other types of towers:
aura towers
super range towers
ultimate tower
life giving tower
gold giving tower, on each hit gives gold