Hello. This is my tutorial on how to build the most compact NPC prison for Terraria. DISCALIMER: I DID NOT COME UP WITH THIS IDEA! I ONLY NOTICED THAT IT WASN'T ON INSTRUCTABLES.

Pros:

Very compact

Minimal build time

Doesn't get in the way

NPCs stay in the prison and don't wander

Cons:

Doesn't look great

Can only have so many before you run out of room

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With that out of the way, let's start the tutorial

Step 1: Build the Frame

Build a hollow 5 x 13 box as shown with two platforms in the bottom, and a way in for while you build.

Step 2: Fill the Inside

Fill the entire inside with walls of your choice, and at the bottom, place a chair and workbench. (It's a workbench because a workbench works and is smaller than the table)

Step 3: Finish the Prison Cell

Place a light source inside and fill in your "door" and your finished!

Step 4: Night

Now wait until night and your NPCs will teleport into their new prison. To access the NPCs, just walk underneath them and right click on them. You can also make them in rows like I did in the Intro, for as long as you have room for.