Some fascinating material here, particularly the 'Environs de ___'
plans with very detailed levelling in and around fortified towns.
Links to these (or some of them?) are given in the original posting.
But I can't work out how to get at them from the Cartomundi home page.
The map-based finding aid requires you to zoom in to exactly the right
area on a very unsuitable modern map before it will deign to tell you
whether it has any large-scale plans in that tiny area. And the
Cartotheque Numerique option, once narrowed down to France, offers
nothing of a larger scale than 1:10,000, apart from the (NLS) trench
maps. Has anyone cracked this?