I vaguely remember a 'win a castle' campaign with the launch of an expansion - AoS? - so maybe they used the space then?
Edit - I'm not meaning the Samurai Empire 'win a castle in Malas' promotion, I've a vague feeling there was another one before that....
And another edit - I was thinking of
this part of the 'buddy' program, back in 2002
"The Ultima Online™ Bring A Friend To Britannia Program"
"2. Prizes; Winner Selection and Notification. One (1) UO Game Castle on UO Game Shards will be awarded as a prize during each of the 14 weeks of this Program (hereinafter, the "Prize"). UO Castles are in-game real estate and Shards represent in-game locations in which to live. There is no cash value to these prizes"