In that case...
Magically speaking, their constraints are as follows:
In order to build anything else, they need a big elaborate stone circle connected directly to the earth. It's possible they'll have to build it mundanely, using the drones; they'd need to give very precise and somewhat bizarre instructions, and might want to have the drones Converted first for mystic resonance reasons, but on the other hand it'll be much easier to Convert people once they have a stone circle, so... tradeoffs. The living room seems like the likeliest spot for it, but depending on other factors, they might choose to use a smaller more out-of-the-way room instead. The tradition is to place the large stone circle at the entrance to one's inner sanctum, but this is by no means necessary, and they should probably not prioritize it.
In order to gather the energy of Devotion and use it to impress building-templates into the world, which is necessary to build a Temple, they need a Shrine open to the sky. It can be enclosed by walls, and even partly sheltered by tree branches or perhaps some kind of shade or lattice, but you must be able to look up from the top of the Shrine and see stars under appropriate star-seeing conditions. It doesn't have to be rooted to the ground, though they're often built that way because it's aesthetically pleasing. It should be somewhere that's easily accessible to their converted Followers for long-term practical purposes, but it's also the most blatantly visibly eldritch of all the necessary constructions, so probably the best place to put it is either a tower room with the roof removed/remodeled, a walled courtyard added to the main building at a side entrance if any are appropriate, or a remodeled outbuilding.
In order to tend to their Cult in the long term, they will need a Temple containing an Altar. The Temple's roof should be open to the sky; the Altar should be rooted to the ground. At home—well, all right, the Lamb has mixed feelings about calling that place home—in their former place of residence, the Temple was a small shed-like building. But (and they're not entirely sure which one of them first conceived of the spark of this idea, but the more they think about it, the more both of them like it) assuming there's a suitable position in the event hall for a rooted block of elaborately carved stone, converting the entire event hall into a Temple might work pretty well. It could be the centerpiece of a revised aesthetic for the whole castle, in a way that would seem completely normal to visitors, as long as those visitors did not come by while the Temple was in active use.
This is all planning for the future, of course. They haven't even sat down to properly think over the question of when and whether to Convert their drones into Followers, and before they do that, the rest is just daydreams. But they're nice daydreams, and practical ones, and it feels good to share in a sense of purpose together.
In fact, it feels so good to share in this sense of purpose that the Lamb searches Paril's memory for any sign of floor plans to this castle, and memories of previous redecorations. How is such a thing usually done? How strange would it be for she and her crown to lock themselves in her bedroom together and draw up plans for potential renovations?