I couldn't make the bridges work convincingly and stylistically with ground elevations that didn't have gentle slopes. This is the 4th or 5th time I've blown out the center of the furball\tankball island to restart. No one just slaps it together with a terrain and calls it good. All the player I've talked to who have built or build terrains go through the same process of building something then erasing it and starting over many times to make it work. Hitech says he like bridges that focus combat at a choke point, so I'll follow his suggestion. His observations on my last terrain helped make the terrain better than I settled for.
I stamped down a 10 mile diameter circle to 25ft. The caldera walls are 7,000ft while those cut faces at the perimeter of the 10 mile circle are 1800-2000ft tall. Each airfeild area has been stamped down to 500ft to reduce the long slope to the 25ft basin. I may just leave those 1800-2000ft walls as is and stylize as cliffs. I've seen rock formations like that in real life. The run off from the airfield I'll blend down from 500ft into 25ft in a smooth transition.
With the ground level around the TT object now at 25 ft, I should be able to create a better looking water system to place bridges over. Building a terrain is a super tanker full of surprises and failed assumptions load of work. Ideas at 25,000ft over the work space that look great very often suck at ground level for reason like it looks like crap, the polygon mesh doesn't want to let you have it, and the one that always trips you up. You misjudged the scale by a magnitude of 100. This is why I'll use part of the terrain I haven't touched yet and test a terrain feature to see if it fits my assumption. Like those bridges....

Here is the blown out center of the island, and once again I have to sculpt in my assumptions about how it will work.