* Caisteal (KAHSH-tul): This translates to "castle," and in many cases, would be the most appropriate word for a royal palace, especially if it's a large, fortified structure.
* Pàlas (PAH-las): This is a loanword from French, and is a more direct equivalent of "palace," suggesting a grand, luxurious residence. However, it might sound slightly more formal or less common than "caisteal" in everyday speech.
Therefore, Caisteal is likely the more frequently used and generally understood term, unless you specifically want to emphasize the luxurious aspect, in which case Pàlas would be more suitable.