This set is geared towards medieval, fantasy settings, but I suspect most of these grammars would hold for a different theme.
This system becomes very interesting once you consider ending building blocks could be replaced by elements that tie closely to your project's vision. At any time you could replace the arches, columns, ornaments, even bricks, by custom components you may have sculpted earlier. What the grammar gives you is the order and structure of these prefabs, but the final look and feel can be pretty much up to you.
As you can see for the moment we are focusing on smaller grammars. You can think of these as smart brushes that will allow to lay walls, floors, bridges, even towers in the locations you choose.
While grammars are able to express entire buildings (even cities), I believe we need to start small and allow you to place these smaller elements following your imagination. Also there is little point in generating an entire castle if the building elements are not interesting enough. So we are making sure we have a solid repertoire of grammars before we take on larger things. Even something as simple as a basic stair tool can save you a lot of time.
As usual let me know what you think. I'm in particular interested in how many of these do you think we would need for a particular theme, how generic or specific they should be, what kind of parameters would you like to have as inputs.