Make the level all one map or use loading screens?

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Oh, that's even WORSE than hardcoded limits.
If you have any idea on how big the functional area is, let me know.

Well, i haven't tried figuring it out, but there is one way to test it. Place a modified grunt whose activation distance has been set to a large amount (e.g. 1000) with triggers disabled (so that it won't attack the player) in the center of the map. The monster will be used to keep track of how far the player is from the center of the map. Make the plane they both stand on just as big as the activation distance (or bigger) and line up a wheel entity every 50 meters. Test the map and find the one that doesn't work. Then find that one in the level editor and delete the rest. Then duplicate the wheel entity 5 times towards the center of the map every 10 meters. Repeat until you know the exact, or close enough, distance.