I always like 1 tile wide corridors when I rely entirely on static defenses like this. Those corridors are hidden passages through the mountain that are only accessible to my colonists. I have them lined with defensive columns just in case.
This is a Caldera landform from Geological Landforms!
I admit that I used Map Designer plus Geological Landforms to create what I wanted. I like to do random formation runs, but this time around I had a plan in mind and needed a specific formation. With Map Designer you can mold GL into basically anything you want. A few river size adjustments, lake placement shifts, and a smoothing of the caldera walls and you have my map.