You will have to remold the terrain carefully, hit g a couple of times to show your medium grid - try to slope the terrain with 3 medium grids - yah it will be a pain and ya it won't as vertical, but it will fix the over stretched rock look.
Roof, suggestion:
Remove the dome, or leave it and add inverted rocks. Invert something like the floating rocks or outland rocks - under 'art - max roll hold down the shift key when you double click, use -3.14 - this will 'flip' the doodad upside down. You might also want to increase the max scale of the rock doodad and decrease its minimum so you can stretch out the x and y and lower the z to get 'flat' rocks.
If you are leaving the dome and are trying for a more lumpy rocky roof, you may want to darken the dome darker than the rock giving the impression of cracks shadows.
Another thing you might want to try is rocks on the walls - this time set max roll to -1.57 this will lay them on their side. You will most likely want the fixed rotation to be -1.00. To spin a doodad select it and hold the ctrl key while moving your mouse with the left mouse button held down.
Yeah you most likely know those things, I insert that data here for others who may read.
Camera - make a camera object, edit its Z-offset you can increase that and 'zoom in' that most likely will improve the image quality. Many only zoom out to get more of terrain in their camera to give height, Z-offset does that without having to sacrifice close ups.
Since you will be using rocks for roof and potentially on the walls, you might want to remove that triangular spike of lava and replace with rocks.
Your structure/platform is ok - However the pillars are too thin and too tall. perhaps using an arch between each pillar, giving the feeling that this is a reinforced structure, or adding a wall or something across?