Reese Heights

There's only one portal and the exit lies to the southeast. The enemy flight path starts from the northwest, near the portal. The first wave will be airborne so place a reaping tower, preferably a holding tower nearby along the flight path. You can block the portal and divert the path to the northeast to force the invaders to take a longer route while enabling you to place some defenses along the flight path of enemy fliers. See the sample layout below. For higher difficulties, more towers will be needed so just construct them accordingly so that you'll end up with a winding path.