Always thought it was Lavos, myself. I figure, they had to have built the fort some time after the Time Crash, since it seems to be a fairly well integrated piece of the main El Nido island. After the battle with Chronopolis, there were obviously survivors, and it may be that some were able to learn about Lavos via their connection to the planet, which would explain why that demo that Serge got included it. Besides, Dragonia seemed to be built along more mechanical and less biological guidelines than Terra Tower, suggesting that the Dragonian's technology had been strongly influenced by that of the humans.