I haven't found much information in the wiki, but from what I see in the history of the orcs they are enemies of almost everyone, so that elf may simply be a prisoner, the fortress is close to Ab. There are many things in tibia that are unanswered, I hope someone knows more.
"Orcs do not have many allies, but they are trusted by the Dragons, and are generally found near murderers and outlaws. They do not need the help of other races for their efforts of conquest, and they have even been known to attack the Amazons without being provoked. "
"Be careful, the King does not tolerate intruders. In the prison cell, you can see an Elf which is not able to do anything and can't be attacked either."
Source:
https://tibia.fandom.com/wiki/Ulderek%27s_Rock
https://tibia.fandom.com/es/wiki/Orcs