Young's Crossing

There's only one exit leading to the south but there are two portals you should watch out for. The airborne enemies will be coming in from the northwest as well. One thing you can do is to block off one portal's path and divert it along the other portal's path so the two enemy forces coming out from them merge. In this stage, the portal to the northwest will be active first so concentrate your towers there.

The northeast portal will activate at the start of Wave 15 so make sure you have ample defenses plotted there as well. You may want to block off the path in the northwest so the enemies coming from the portal there is forced to divert towards the northeast portal and more of your defenses.

Don't forget the waves of aerial enemies that will come in from the northwest. Lining up some Skywatch Towers is fine though you may want to put in some Holding Towers nearby to help slowing down airborne and ground targets.