When it comes to describing a sinister and creepy castle, there are several synonyms that can be used instead of "horror castle". One option is "haunted castle", which implies that the castle is occupied by ghosts or spirits. Another synonym might be "spooky castle", which suggests a sense of unease or fear when approaching the castle. "Gothic castle" is another possible alternative, which describes a castle with dark and mysterious architecture. "Malevolent castle" could also be used, indicating that the castle has a harmful or evil presence. Other options might include "macabre castle", "sinister castle", or "terrifying castle". Each synonym offers a slightly different nuance or tone to describe the ominous and forbidding presence of a castle.