An enclosed space is a gap surrounded by blocks, Level 5 is the first place to get this, if you try to make as many gaps as you can without the building collapsing you should get it within a few tries.
Update: thanks to Senior Chupon
This is super easy on level 9. You start with several narrow vertical walls that you can easily cap off with a flat piece.
Update 2: Thanks to SPIRITWRAITH
this can be achieved on lvl 1. I completed the first few lvl's then went back and replayed lvl just to get this achievement. Unlike lvl's 4 and higher, lvl's 1,2, and 3 have unlimited blocks and a low target scores, so you can easily get 20+ enclosed spaces and complete the lvl in about 1 min. Also works on the WP8 version