I'm hoping to find one of the larger skeleton forts. Found one or two small ones, but not a large one yet like I ran across on my last world.

Due to the terrain system what I made was a small shack on a peak first. This ended up a central tower with windmills.

I then dug around trying to find a good level where I could make it mostly flat.. basically chopping off the top of a mountain but having a central hump of unmovable earth.

https://steamcommunity.com/sharedfiles/filedetails/?id=2462663810

Just starting in this one with the center "cabin" on the top peak.
https://steamcommunity.com/sharedfiles/filedetails/?id=2462664007

This is when it was nearly done and had found a good lower level to develop.
https://steamcommunity.com/sharedfiles/filedetails/?id=2464016506

Nice thing of being at the peak is you get sunrise and sunset views.

I also found it nice to have water near by to get the sparkle off it.
https://steamcommunity.com/sharedfiles/filedetails/?id=2567777556

In terms of a basement ... you may be better off finding a huge boulder to hollow out. Most of the basements I made were very terrain specific. Noticed if you have any part of a floor block in dirt it will stay up. If you have one on each side of a gap you can sustain a single block unsupported but it needs to be built in a certain sequence.