Been doing some digging on good old google and playing with lines on the map. Bordeaux is south east of cherbourg. It’s a direct line of site from cherbourg to angers to Bordeaux, and the have a cathedral where the only original surviving structure is the nave. They protected it and rebuilt the cathedral around it.
Contrary to that nef was translated to nave as a literal meaning. But nef is also a paper fold meaning a ship. It originate from ancient French which he has been using all the way through this clue. May be a port directly south of cherbourg as this would cross the water twice (neptune’s help).