New "Work" option: Fill (opposite of dig) to raise elevation of ground
Add a way to raise ground, at least in mines that have been dug already. Presumably to fill in with dirt. Not sure if you'd need a dirt resource to do this (my impulse would be "no")
EDIT: a suggestion below where filling it in would cost stone makes more sense then the introduction of a dirt resource.
I just dug around in an oval almost and basically turned the slopey hill into a cliff fortress. Medieval castle building 101. They also did it in roman times, such as at Masada.