Thats a point, I always think of Ebonheart as a city, when its really a castle! Cheers!
But Ebonheart (the castle version) is also a city.
Old Ebonheart is just the a name used to make a distinction between the two Ebonhearts.
Old Ebonheart is also the ancestral home of the Ra'athim clan.
From "Madness of Pelagius'
There are two reasons most scholars give for the union. Magnus was trying to cement relations with Ebonheart, where the Ra'athim clan hailed. Ebonheart's neighbor, Mournhold, had been a historical ally of the Empire since the very beginning, and the royal consort of Queen Barenziah had won many battles in the War of the Red Diamond. Ebonheart had a poorly-kept secret of aiding Uriel III and Potena.