What are the minimal requirement for building non-enemy spawnable regions?

I just started playing Terraria for iOS (yeah, I know I'm late to the party) and I'm wanting to build a HUGE safe structure that's both above and below ground.

I'd like to clear out the area and make a barebones structure, first, and later I'll fill any additional materials, furniture, etc.

What are the minimal requirements for making a region, for lack of a better term, where enemies will not spawn?

Solution 1:

Make sure the area in which you want no enemies to spawn is closed up, with walls and light sources around. That should make sure no enemies spawn. Any building that fits the housing requirements works as well.

*Most important is the walls