If it freezes. Else, you could just use pumps in an attempt to keep the area dry for enough for your masons to build. Or MAGMA of course.
It DOES freeze, just not for very long. a month to a month and a half, to be more precise. Right now, though, I'm contemplating how I will use this very small window of opportunity to be most effective. I'll have to move the block designation closer to the build site and immediately start building downwards, first walling off the region in one season, then putting in the floors the next season.
Finally, I MAY put in Magma into the lake.... if there's even magma in this area. My first concern is to erect the fortress.
Here's a high view of what I have so far. The main path will connect to the northern section, and will function as a path for the trade caravans. it is 7 tiles in width. The smaller paths are 5 tiles wide and are major walkways for the dorfs, but still wide enough for wagons to travel *not that they would need to*
The unwalled region will be the main structure, where all the workshops and storage designations are located. The walkway to the west will lead to the residential areas (for normal dorfs), a tower for all the nobles, and a barracks for the military. Once the King arrives, I'm going to build a separate castle for them.