Imperials are permanently at the following camps:

Dragonhead
Crooked Fork (near the Mark of the Lover)
Drybone (near the Mark of the Warden)
Broken Water
Iron Lake
Crimson Bark

If I missed any, please feel free to mention other places. I'm not sure if it has to do with the Twelve stones, but my first intuition was that they're trying to sabotage our only hope. It could be the natural spread of Imperial territory, but why would they bother occupying so remote a location as Gwyr-Aen, let alone Crooked Fork? The Mark of the Warden is in Sandgate, too, which is rather remote. I haven't checked Lynxpelt, but all of the other Twelve Stones (save for those of the Wanderer and Builder, which were already guarded) have no Imperial presence.