Stained glass windows on The Throneroom of Cedrica
I see that this popped up on HNQ, so I wanted to expand on shoover's excellent answer with a build showing one way that this could be put together for folks that may not be as familiar with LEGO elements as the Bricks community.



As was already pointed out, the key element here is the 2x2 turntable base without a turntable attached:



Here's how you might pair those elements with transparent 1x1 and 1x2 elements to build a stained glass window:





We leave a couple of 1x2 plates overhanging the back to provide attachment points.

Here it is from the front. Notice that we also add a couple of 1x1 plates on the front side to make the connection points flush with the rest of the window:



Now we can build a frame to trim this out and create a nice looking window:







We can then attach our window using the attachment points that we left for ourselves in the back side of the Erling bricks:





Note that I'm not trying to replicate the exact techniques used by the original designer, but this does show one way to get a similar look. I've added the parts list on Rebrickable if you want to see exactly what I used and build this yourself.