Waves

In Minecraft Tower Defense 1 and 2, waves of mobs from the Overworld to the End come after you. Each wave has a special kind of mob or mobs. Here is a list on the waves and the types of mobs in them:

Wave 1: Zombies & Skeletons

Wave 2: Spiders

Wave 3: Creepers

Wave 4: Zombies & Skeletons

Wave 5: Ghast

Wave 6: Silverfish

Wave 7: Zombies & Skeletons

Wave 8: Endermen

Wave 9: Slimes

Wave 10: Spider Jockies

All waves will be the same in each 10 waves (ex. 1 and 11, same. 9 and 19, same). Also, the ammount of mobs and damage they can take is increased every 10 levels.

Strategies
An easy way to defend your base in MInecraft Tower Defense is to have very long paths to take the mobs longer to reach you, have dispensers at good levels (blue) to attack more, and lava pools take big chunks of life out of mobs.

Waves (Minecraft Tower Defense 2)
In Minecraft Tower Defense 2, waves are limited to 5 or 10 waves in each area. This makes the games a little faster but there are usually many mobs. If you want unlimited waves,you have to beat Adventure Mode to unlock Survival Mode.