It was where Jabba lived, and where his evil goons would reside as well.
The reason why Jabba lived there was because Tatooine was sandy, and he enjoyed taking trips to the beach.
The castle in question was always filled with Jabba's goons, meaning that no enemy of Jabba would survive long in there.
Jabba usually sat on a throne in the castle, along with his
sexy slave girls, which he did kinky things to every 15 minutes.
It was also filled with nasty traps. There were trapdoors that would make people fall into a pit, where the nasty Rancor resided. Usually, they died within a minute or two...