The following images are textured versions.
First thing I did was create a base using the cube from the Probuilder and just extruded it to the size I desired.
Next thing I did was make a door and luckily Probuilder already had it so I just resized it to fit the base and the walls which I created using the same method as I did with the base.
The next thing that I did was expand it so I made a corridor and then I made two cells out of the second room.
Here you can see a bed of hay and some cell benches. There is also the cell door which i had to precisely resize so that it would be the same symmetrical to the walls from top to bottom. All of the assets that I've used here are all from a dungeon asset pack that I got from the Unity assets store for free the pack that I used was called Decrepit Dungeon.
After the second room I wanted to keep expanding so I made new corridor leading to the third room.
For my third room I thought of making more cells but then I thought of it being a level so I thought that I would put a little challenge for the player so then I thought of making a maze for players to figure out where the exit is. To make it not as hard I thought of putting fire pits along the way.How I made the fire pits was that I used the cylinder shape from Probuilder then I scaled the top part to be wider and then I extruded it downwards to create a concave surface to make it look more like a pit.
For the fire part I just removed the fire from the torch asset which came along with Decrepit Dungeon pack then just move the torch downwards to where its not visible because i found that if you delete the torch the fire gets deleted with it.
When I took the print screen of this the lights weren't done rendering yet so the actual lighting is much brighter although it makes it easier to move around the maze because you can see where you're going you would still have to look for a way to get out of it which is still a little challenge.
Once you find the way out of the maze you will see stairs that lead up to the fourth room. Like the maze, when I took this print screen the lighting wasn't finished rendering yet so it's a bit dark but in the forth room you will first encounter crates and barrels along with the guards desk that has candles and the cell keys which also comes with the decrepit dungeon pack from the assets store. But as you go deeper you will find more jail cells. 
After the fourth room there is the fifth room which is basically just all Jail cells but the corridor leads to the stairs that then leads to the sixth room.
The sixth room is the torturing room although it's not very visible, the torture room consists of hanging cell cages, chains, fire pits under the cell cages, torches and tables and chairs for the ones torturing.
After the sixth room there is a corridor leading to the seventh room which has the underground jail cells which is also another kind of torturing device.
This is the finished version of my 3D level, here you can see the difference in the lighting but near the end you will see a couple of texture errors on the walls of the underground jail cells.