It's taken me four hours, but I've finally finished transferring my gridded image of the fort to a 3 foot by 4 foot board of 5mm MDF.
The next step will be to start building up the fort from polystyrene.
Regular commenter DC wrote to ask about scale for the French Castle and I started rambling. As you would; It's early in the morning here and I've not yet finished my first cup of coffee. I'm dropping my reply here.
I'm going to start off as I did the base-board and look at keeping to a 1"-equals-10-yards standard and see how I go from there.
When I'm building a piece of wargames terrain, I telescope the scale of the object depending on how large ther are in reality.
For example, I recently built three model ships, each of different sizes. The smallest (a Torpedo-boat) was a straight scale model at 1:60, whilst the others kept their 1:60 vertical scale while their length and width were more and more ruthlessly truncated to make them "gameable".
I think by that measure, it depends on what you want the buildings to do. The smaller the scale of the game (ie, skirmish), the larger you want your buildings to be so your little men can interact with them more. In larger games the buildings should become more abstracted, while fewer of them ought to represent more substantial settlements - 4-5 houses can be a town, 7-9 a town and so on.
The French Castle (or machicolated house) will get that sort of treatment and suffer some scale compression on the 'ground', but will still be fairly imposing.