You can basically already see the path the enemies will take from the beginning of the game. Use this knowledge to plan accordingly. Some stages will give enemies multiple locations to spawn from, meaning that while it be tempting to stack a bunch of fortifications at the entrance to one, it's not a good idea in practice. These paths usually converge, and it's there where you should place a bottleneck. Orcs are good to slow down crowds, while you use other defenses to finish the enemies.