I had some time and added some columns and ribbed vaults:
The way it works, actual architectural elements can be swapped at generation time. This means different types of vaulting, columns and ornaments can be used without having to change the grammar.
The grammars are modular. What you see here is the result of one particular module that is able to fill up any space with vaults and columns.
I still have some issues in the radiosity solution. There is some light at the top of one vault I cannot really explain.
The mood in this screenshot is set by post-processing tone mapping. This is not something I can do in realtime yet. I plan to cover it as soon as I start working on the client.
A few minutes later I found the bug in the radiosity. Here is the same vault with improved lighting now:
That was quick.