This is the Instructable for my Rat Castle Cubes listing on Etsy. The files are available for $5 here:

https://www.etsy.com/listing/640920092/rat-castle-cube-digital-file?ref=shop_home_active_1

The design is modular at the level of each cube so you can rearrange them to create different castles just as if they were Legos.

This is a difficult and time consuming project. You probably shouldn't attempt it unless you are reasonably handy.

Step 1: Choosing a Material and Thickness

The files for this project work with acrylic or wood of 3 thicknesses: 4.8 mm, 4 mm, and 3.1 mm. 4.8 mm is roughly equivalent to 3/16 inch and 3.1 mm is roughly equivalent to 1/8 inch. I built my castle mostly out of 3/16 inch plywood with just two cubes of 3 mm acrylic. If possible, you should choose 4.8 mm materials because the increased thickness will make your cubes more robust. Only use the other thicknesses if you don't have access to 4.8mm, (3/16 inch) materials. If you can afford it, acrylic is probably the better material. You will be able to see the entire interior of the castle and it will be much easier to clean. The main downside to acrylic is that it is significantly more expensive than plywood. It is also a little more difficult to work with. At the very least though, the doors on your castle should be made of acrylic so that you can see inside. The acrylic doors can be any thickness between 3 and 5 mm as long it is no thicker than the material the rest of the castle is made from. If you don't want to buy acrylic, you can just have a wooden castle without doors.

Step 2: Choosing Files

Unzip the file from Etsy using Winrar or similar software.

The files should appear as they do in the photo above. The files are in both .DXF and .ai formats. Choose the file that corresponds to your preferred material, thickness, and file type. For instance, I would choose 4.8 mm Wood Panels.ai. If you're sure you don't want doors, you only need one file. If you do want doors, pick the Acrylic_Door_(can be 3-5mm) .DXF or .ai file. The door file is separate because it will always be cut from acrylic whereas the others can be cut from acrylic or wood. Open these two files in your preferred vector editing program. I used illustrator, but free alternatives such as Gimp will also work: https://www.gimp.org/. If you use the .DXF files make sure that you import them with mm as the units.

Step 3: Manipulating Files

The raw files contain one of each of the panels that comprise the castle and should look like the first photo. Don't cut from this file directly! Instead, manipulate the files to create boxes.

The second photo is a box with a solid floor and ceiling, a doorway in the front, a wall in the back, a window on the left, and an archway on the right.

The third photo is an example of a right handed staircase cube with passageways through the floor and ceiling. It has an archway in front and back, a wall on the left, and a doorway on the right. The three loose pieces in the lower right make up the stairway. Don't worry if this looks confusing. There are assembly photos for each type of block in the coming sections.

The fourth photo is another right handed staircase, but instead of an archway in the back, it now has another wall.

The fifth photo is a ramp block.

The sixth photo is a platform block.

The seventh photo is a half-platform block.

Play with the files until you have a good idea of what types of blocks are possible. Keep in mind that each block other than the platforms and ramps should have a doorway that you can access. This is important to ensure that you are able to retrieve your rats when playtime is over.

Browse through the rest of this Instructable and then return to this step. Once you feel that you're ready, create a file that contains all of the pieces you will need for your castle. Keep in mind that if you want doors, you will need to include them from the Acrylic_Door_(can be 3-5mm) file. If you are cutting everything from acrylic, you can add the doors to your main cut file. If you're using wood for the castle though, the acrylic doors will have to be cut from a separate file. Don't forget to include one acrylic door for each doorway.

Step 4: Cutting Your Panels

Hopefully you have access to a laser cutter. Cut out a few pieces and make sure that they fit properly. If they don't, you will have to adjust the kerf on your laser: https://www.instructables.com/id/How-to-Adjust-for...

If you can't access a laser cutter, you can order your pieces cut and shipped to you online. If you do this, make sure that you only buy a few pieces first in case they don't get the kerf right. It would be terrible to order hundreds of pieces only to find that none of them fit together. If the first batch is OK, go ahead and order the rest.

Step 5: Ordering Supplies and Hardware

You will need the following items and tools in order to assemble your castle. If you're making the castle out of wood, (with the exception of the acrylic doors), you won't need the acrylic glues. If you're only using acrylic, you won't need the wood or super glues.

If you decide not to include doors, you won't need the hinges or clasps.

You can purchase the carriage bolts and wing nuts at your local hardware store if it's more convenient. Just be sure to measure the square part of the bolt head. It needs to be within .5 mm of 5 mm.

There are lots of other hinges and clasps available on amazon, but if you use them, even if they look similar to those listed below, it is likely that the holes in the door and doorway will not line up properly. To ensure a proper fit, use the hinge and latch listings below.

Etsy Files: https://www.etsy.com/listing/640920092/rat-castle-cube-digital-file?ref=shop_home_active_1
3/4 inch long carriage bolts: https://www.mcmaster.com/93548a109
Wing nuts: https://www.mcmaster.com/94225a320
Thin super glue: https://www.amazon.com/Bob-Smith-Insta-Cure-Super-...
Hinges: https://www.amazon.com/gp/product/B01EMQZA98/ref=o...
Latches: https://www.amazon.com/gp/product/B078LY7ZKC/ref=o...
Thin acrylic cement: https://www.amazon.com/Weldon-Applicator-Bottle-Pi...
Thick acrylic cement: https://www.amazon.com/SCIGRIP-Acrylic-Cement-Low-...
Elmers or wood glue
Small Philips screwdriver
X-acto knife (not strictly necessary, but helpful for trimming pieces that don't want to fit together.)

Step 6: Basic Cube Assembly

I wanted a cube to fit in the gap in the first photo. It needed an archway on the left, a wall in the back, a window on the right, a door in the front, and an open floor to accept the staircase of the cube below. Since the new block is not a staircase itself, it had a solid ceiling.
I spread out all the pieces as in the second photo. Each side piece has two notches on one side and three on the other. Make sure that they align properly.
Assemble the door according to the Door Assembly step before returning to Cube Assembly. If you aren't using doors, continue to step four.
Once the door is complete, push the panels in place one at a time as in the photos. If you prefer Elmers glue over super glue, you can apply it to each panel before you press it in. If you have super glue, you can apply it to the seems after the entire cube is assembled. In my experience, the super glue is much easier. If you are using acrylic instead of wood, you should use the thin acrylic cement to hold the panels together.

Step 7: Door Assembly

Skip this step if you aren't using doors in your castle.

Gather the pieces shown in the first photo.
Decide whether you want the door to open to the right or to the left. The second photo shows a door that will open to the left. Rotate the plexiglass by 180 degrees to get a door that will open to the right. Keep in mind that left opening doors will have upside down latches. They will still work, but this might present a cosmetic issue.
In the third photo, I decided to make a right opening door. I used the long screws that came with the latches to attach the two latch pieces to the door and doorway.
Since the screws are longer than the plywood and acrylic are thick, I stopped when the screws were just beginning to poke out as in the fourth and fifth pictures.
I placed the widest of the small wood pieces on the back of the doorway so that the holes lines up with the screws that were barely poking out. I then tightened the screws all the way into the block. This ensures that the sharp ends of the screws will not scratch your rats. The wooden pieces also prevents the door from opening inward.
In the seventh photo, I used the same process as step 5 to attach one of the long, thin pieces of wood to cover the screws that were poking through the acrylic door.
The eighth photo shows the placement of the hinges. The hinges come with their own screws which are smaller than the latch screws.
In the 9th through 11th photos I attached the smallest wooden pieces to cover the hinge screws.
In the 12th and 13th photos I screwed the other ends of the hinges to the doorway.
This time the screws just barely poked out enough to scratch, but not enough to hold the wood. I used super glue to hold the wood pieces over the ends of the screws.
The penultimate photo shows the finished door. Notice that it has been rotated 180 degrees to make it open to the left. Most of the panels can be rotated or flipped without changing their function. The doors and the two staircase panels are the exception. Rotating a door will reverse the direction it opens. Rotating a staircase will reverse the direction of its spiral. Since most of the panels have two notches on one side and three on the other, rotating one panel in a cube layout will require you to rotate the rest to match it.
The final photo shows leftover pieces. You might not need all of the wooden screw guards.
Return to Cube Assembly

Step 8: Staircase Cube Assembly

Staircases can spiral either clockwise or counter-clockwise. The first four photos show a clockwise staircase. The last four photos show the same panels, but reversed so that the spiral is counter-clockwise.
Even if you are using super glue, it might be a good idea to use Elmers glue as well on the interior staircase pieces in order to fill gaps between the pieces that are too wide for the thin super glue. If you are using acrylic, you could use thick acrylic cement instead of the Elmers and thin acrylic cement instead of the super glue.

Step 9: Platform Assembly

Platform assembly is straightforward. The half platform is not pictured, but is assembled similarly with two side pieces instead of four.

Step 10: Ramp Assembly

Do Not Glue The Ladder Piece! Assemble as shown in the photos and glue every piece except the ladder one. This has to remain loose so that it can be removed to access the hardware that will hold the cubes together. Note that the wall is not a standard wall. It has a flat top instead of notches.

Step 11: Castle Assembly

Once you have a few cubes, start putting your castle together!

Keep in mind that you will need to be able to extract your rats at some point, so make sure that every cube has a door that you can access. Each cube should be held to the cubes that are adjacent to it by a carriage bolt and wingnut. The second photo shows where square holes aligned on the left, right, and bottom of the cube so that carriage bolts could hold the cubes together. All the cubes, even the special ones like ramps and platforms are held together this way. As long as your pieces are well glued, this system will make your castle rigid and strong. See picture three for a stress test. Now that you've finished assembling you castle, you're done! Let your rats have fun and if they ever get bored, just rearrange the cubes to give them something new to explore.