Walls

“	"Defensive structure used for fortification purposes. Blocks the path of ground units. Effective when placed one after another."	”
	— in-game description

About

If placed correctly (in front of turrets) walls can prevent enemy units from attacking your turrets.
Moles can bypass walls due to their attack tactics (tunnel around walls)
Walls are one of the few things in Galaxy Life where 3D physics are taken into account. Level one walls, since they're short, cannot prevent Beetle Tank bombs, whereas level two and above walls can. Bazookas can shoot over walls no matter what unless they are extremely close to the wall.
Building and upgrading walls give off good amounts of exp. The price of upgrading walls increases exponentially as the level increases.ot over it
Very useful against S-Trikes. a completely surrounded base could block the strikes out of it
Pay attention! Upgrading Walls beyond level 3 costs a lot of coins. Pay attention to your money!

Kamikazes specialize in destroying walls, so it's recommended that you place Compact Houses or Mines in front of walls, as to put more space between the unit deploy zone and walls.
Moles specialize in bypassing walls. If you want to stop them, you can put walls in spaces in your base, as a way to prevent Moles from entering.

To see detailed info on how to unlock each level, and the total amount of buildings per level visit: Building Level Unlocks at Star Base's Page.