Greaseworks
Four Days: World Defense Guide
The portal will be in the middle of the map with one exit. The enemy flight path will start from the southwest as well. You can use the objects in the map to block off the path and force the enemies to move around from the southwest to the northeast. You can refer to the screenshot below to know the initial tower setup.
At wave 9, the first batch of enemy fliers will arrive so make sure you have anti-air defenses and preferably, a holding tower along or near the flight path. Once you've successfully established your anti-air defenses, concentrate on upgrading your dual-purpose towers (can target both air and ground units) near the portal and along the flight path.
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