I noticed during a All Gaul Sandbox playthrough that there's a fort on the path on the north side of Aquae Bormiae (between Maiensis and Brigantion) that is unable to be built due to being inaccessible terrain. No, it is not due to winter. Tried with both a Legio and a Salassi Skirmisher, both displayed the red text Terrain Inaccessible when trying to build it.

Just a map mistake.