terraria why are monsters spawning in my house?

Answer 1

technically you need to add a backwall, not the dirt one thats in the top layer, and if you can see the sky through it, mobs can also spawn

gather some wood, stand by your workbench and you will beable to craft a woden wall, 4 parts for every wood, now cover the back to make it safe

to get rid of the dirt backing use a hammer