But ... but ... what does Halicarnassus have two of? You don't say and we must know! Horns? Other anatomical bits and bobs (you know you were thinking it)?

Sorry but the apostrophe in there set off a twitch in the grammar part of my brain.

Halicarnassuses is the proper pluralization.

Also Halicarnassi would be incorrect because Halicarnassus is of Greek origin and not Latin.

Anyways, forgive the tangent.