I've been reading Castles, Battles, and Bombs by Jurgen Brauer and Hubert van Tuyll, and economic approach to military history.

Based on this, the most basic castle, a solid stone keep with a curtain wall would cost 2 GB for a minimum of 8 seasons with a possibility of one or two extra seasons of construction. It would have no maintenance (in fact it should increase tax incomes) basically functioning as a level of law holding.



To build a modern castle, with all of the things that make a medieval castle a highly defensible place, multiple walls, numerous round towers, water barriers or other difficult locations, with multiple gatehouses, would take closer to a decade to build at the cost of hundreds of GB.

Builth was a castle centered atop a motte which supported a great round keep and was enclosed by a masonry wall defended by 6 towers. Two earlier Norman baileys remained as part of the complex and so don't figure into the cost. The whole thing is encompassed by another curtain wall and accessed through a twin-towered gatehouse. Other structures included a kitchen block and the great hall, a chapel, and residential quarters. Apparently, construction was stopped at Builth in 1282 although the work on the gatehouse may not have been complete. It took five years of construction and about 85 GB. That's about 4.25 GB per season.

Aberysteyth cost 195 GB, Rhuddlan 475 GB, Harlech 500 GB, Caernarfon, which included a walled town as much as 1000 GB or more.

Aberysteyth took 12 years to build costing 4GB per season
Rhuddlan took 8 years costing almost 15 GB per season
Caernarfon took 12 years costing almost 21 GB per season

So the fort action must involve substantially smaller fortifications.

One wonders what kid of fort level Caernarfon would be.