Hooktail Castle is a place in Paper Mario: The Thousand-Year Door.
As the name suggests, Hooktail resides in this huge castle. Until her defeat at the hands of Mario, anyway. Mario must journey here to find the first Crystal Star, the Diamond Star, which is being guarded by a multitude of enemies, along with the dreaded dragon, Hooktail. With the help of his new-found partner, Koops, Mario can hit switches that require his shell attack and "hold" move to activate. While he is searching in the castle, he learns the Paper Move in which Mario obtains the strange ability to turn sideways, causing him to be able to slip through bars and obtain many previously unreachable items, such as the Attack FX R, badge, an item vital to the defeat of Hooktail.