Orcs are large ground units, and you'll need to be sure to leave plenty of space. Orcs require a gap as wide as one of your towers in order to fit through. If they can't find their way through, they'll begin destroying any towers near them to try and open up a path.

Smaller than orcs, trolls only require an opening half as wide as one of your towers in order to pick their way through. However, they can be more dangerous if blocked, because their smaller size and greater range means that they can destroy your towers even more quickly. Trolls and orcs move at the same speed.

Like the orcs, mystics are large ground units, but they have the additional ability to phase shift when threatened, blinking out of existence for a short time after receiving damage. This can disrupt your targeting, and also allows other targets to pass them while they're phased out. Mystics do not continue to move forward while phased. Mystics are the same speed as orcs.

Though not quite as fast as the ground targets, wyverns easily make up for it with their ability to fly, passing straight over your towers as if they weren't there. Wyverns can also pass each other in the air, and so don't get caught behind each other either. You'll need to position your towers carefully and make good use of cold and venom towers.

Flickerbats combine the wyverns' flight with the mystics' ability to phase shift, making them singularly hard to corner.