I was playing a game recently and i abandoned a sea but i put a power chip in it. One of my opponents then mustered a ship into that area. Is that allowed to do since i didn't have any ships in that area or can they only muster a ship into areas that are empty AND not owned. This blunder ended up in me losing pyke and the game so I would like to know for next time.

You cannot establish control of sea areas, only land areas.

So no, you cannot muster into an enemy territory, but sea areas are only enemy territory if they have a ship in them.

Thanks, I didn't realize that.