The Floral Lab was originally designed as a facility for study of Flora and Fauna, and had minimal access from the ground level. Rather, a hidden entrance was devised to keep the place secretive, and documents speak of odd experimentation being the prime focus.
One such example occurred in 502. The village of Rhad-du-cet, which borders the Deep Marsh, was overrun by Shambling Mounds and it's entire population decimated. In 504, the Sage Miros Chanel discovered that the entire population of the village had been infected with the Shambling Curse by a contagion that was created in the Floral Lab. All research in the facility ceased immediately and the building remained empty for 7 years while Miros and the Council of Crestfallen dealt with the plants and neutralized the facility.
In 528, work began to expand the lab, and it reopened in 533 as a prison for the vile and hardened criminal faction. No entrance above ground lead into the prison from within the castle proper, although there are unconfirmed rumors of hidden passages.
Presented below is the Prison Complex: