Zerg Spire
The only obstruction in the middle of the map is a tall zerg spire on an isolated plateau.
Natural Defense
Destructible rocks easily defend the natural expansion of each player, the only other way through is through the main.
...To my base
As you can see the Zerg fit right in...
Protoss defending main
Protoss defending main from terran marauders. Also hopefully you can see the custom lighting . There's an orange tint, and the bloom is turned up just slightly to make the lava shine! Looks better in game.