You are allowed to set up rocks to sit on as a bathroom.

Chapter 313 Section 1

Chazal have many different laws as to what looks more like building and therefore when making a closure for a window or making door bolts etc., you would need to show before Shabbos that it is part of the house. A question should be asked if you are adding to something, whether it is considered building or not.

Chapter 313 Section 2

Putting a peg into the door and ground is stricter unless the holes were made before Shabbos.