Considering the Nazis actually occupied the place during WW2, there's actually a lot of untapped potential here and a lot of horror you can do.
During World War II, the Wehrmacht occupied the castle until 1945. The Nazis were said to have conducted experiments into the occult. According to one source, there were "multiple myths about their supposed occult involvements there". Another source states locals believed that the Nazis had been using the "powers of Hell" for their experiments.
In general, there's a lot of weirdness about the place that has always fascinated me, like its surroundings. Granted, it was apparently built as an administration center for royal estates but the locale is weird.
The castle was built in an area of forests, swamps and mountains with no external fortifications, no source of water except for a cistern to collect rainwater, no kitchen, far from any trade routes, and with no occupants at its time of completion.
And the feature people mention the most when talking about the place.
Houska Castle, and most specifically the chapel, was constructed over a large hole in the ground that is a "gateway to Hell", which is allegedly so deep that no one could see the bottom of it. [...] The pit in the lower levels of the castle is said to be a gateway to hell. Thus, by constructing the Gothic building's defensive walls facing inward, they were able to keep the demons trapped in the lower level's thickest walls closest to the hole of the castle.
I'm not smart enough to come up with my own lore but I'd be so stoked if someone actually tried this considering Houska Caslte is never really talked about at all.
Since it's in the Czech Republic, you don't have to rely on Christian depictions of hell and demons but you can tap into Slavic folklore.