I did another estimate, this time based on Chateau Galliard, which was built with remarkable speed in one-and-a-half years with 6000 workers. The stone was local. King Richard oversaw the building himself on site and drove them hard.
I measured Chateau Gaillard in photoshop, counting the towers as circular wall of said circumference, and came up with 467 meters of stonework tending to be 3 meters thick and 10 high, plus one Great Keep of unusual thickness and height.
We only want 6 meter high walls, but we want it 40% longer. That results in an overall volume of stone 84% of the Chateau without the Keep. Add the keep as our one allowed tower, and that's 1.25 years of work for 6000 men.
---
Okay, then. Let's set all this building in motion.
The KeepA massive tower riverside on the downstream approach from the Count's lands:
20m tall, 16m inner diameter, 19m outer diameter.
6 storeys, 200 square meter floor area, 1200 square meters total
One floor would be enough to hold our garrison in a long siege, and our current town population of 200 could hide on two floors during a temporary emergency. This is a worthy keep, and a little larger than the Great Keep at the Chateau.
Time: ~3 months for 6000 men.
The Wall6m tall, 3m thick, topped with
bartizans.
640 meters long, enclosing three sides of a 250x200 plot, which can fit about 800 people in two-storey half-timbered dwellings, plus market and public buildings.
Time: ~1 years for 6000 men.
---
The secret is getting 6000 men, so crash construction with all of our revenues spent to bring outside workers, and all of our indentured inhabitants from the countryside (about 900 peasants) providing service on this when not in the fields.
Now the bright side, bringing that many people here ought to jump start the local economy and make our autumn fair a great rollicking crowd.