Found castles (e.g. Firbolg fortress) and built

[Sedna]

In a current MP game I'm playing Oceania with a Kelp Citadel. I begin construction of a second fortress in province X. Next turn I search province X and discover a Kelp Fortress, which is supposed to be +1 Nature and a Kelp Fortress. I get the gem, but no free fort. I'm guessing that when my castle is done building I will get a normal Kelp Citadel, but I'll have to wait and find out.

Does anyone else have information on how this works? I know you can't build a fort in a province with a magic one. I assume if you have a fortress already built in a province and discover a magic fortress (Kelp Fortress, Firbolg Fortress, Castle Arcanum, etc.) the magic fort doesn't appear. What I'm curious about is what happens if I stop building my Kelp Citadel. Will a Kelp Fortress show up next turn? What happens if an enemy were to take this province (this is the only way I know of eliminating the "Continue construction" command)-- would they get a Kelp Fortress? What happens if I demolish my Kelp Citadel, does a Kelp Fortress magically appear?

Anyone have experience? If not, I shall have to start SP games with lots of magic sites and a rainbow pretender until this unlikely occurrence happens again.