I quote MuseOD, seeing as s/he posted this idea a while back in a different thread 
Here is a pic of Kew Gardens, the largest (I think?) greenhouse/s in the world. It’s about 21 hectares in all, in Surrey, England.
Now, here’s a photo of what Kew Gardens looked like in the Victorian era
Rather grim compared to the modern version, isn’t it
And it would be even more so in GD, seeing as it would be in ruin/half-broken down :eek:
Also, I reckon a greenhouse interior would be an awesome area to fight in, as it could have both narrow walkways with enemies every few feet, or large roundhouse-areas with both melee and ranged enemies all around. And imagine fighting in an area like the one below, except all the plants would be dead/corrupted due to the Cultists’ rituals, etc.
Hope you all like this idea! 
Cheers,
Younghappy 