I have found out how to build a simple pyramid that also allows you to place object inside and allow sims to walk in and out without trouble. No mods or cheats are required!
Step 1.: create a box with all sides being the same number of walls (eg. 40×40), and put door(s) in the middle.
Step 2.: Now you have to put some roof down next to the walls. Choose the second roof type and leave some tiles empty between the walls and the roof's starting point (I personally chose 3 tiles), and put down the roof on three sides (except the front side).
Make sure to have the next roof's starting point at the end of the previous one so that they blend together nicely.
Step 3.: On the front side, do not pull the roof all the way until the door (I left out 1 tile).
Step 4.: Now choose the first roof type to finish it off. I left out 1 tile next to the door and made the roof 4 tiles wide, making sure it blends into the other roof.
Step 5.: Put a regular roof on top of the box building.
Step 6.: With roofs down, it should have some excess walls visible. They are necessary byproduct of these roofs. Paint them.
You are finished.
As you can see, this will not create a seamless texture. But you can choose another roof texture that blends together easily. It needs a texture that has no different coloured "top edge" part. Using custom roofs is also an option.
We needed to put down the roofs this way. If you only put down the regular roof then you couldn't add a door and couldn't put down objects near the edge of the structure, thus having to make the rooms smaller. But this way you can put objects anywhere within the rooms. You can only not put them under the roofs.
This way it is functional, sims can also walk in and out of it.
Thanks for reading my tutorial. I apologise for mistakes as my native language is not English.