All I've decided is what Weft's described as seeing, namely that there's living turf and plants inside the Hall. Don't be afraid to make stuff up. :)
To assuage Sylvie, let's say that if not actually growing there, it's at least been transported gently enough to be replanted somewhere afterwards.
And there could well be animals in there too - although, to assuage Weft, if there are, they will have been screened for rabies.