instead of vanishing, the should/would suffer damage if unoccupied. The simple pallisade forts of current disintegrate (or they did in RTW, haven't built any in MT2W), because logs fall over, rot etc with no one around to repair them. A sturdier fort (wooden castle for example) would last longer, but perhaps gaps would appear in the walls over time. A stone fort is inplausible as it would require a huge amount of effort to quarry stone, and these are supposed to be temporary structures. Founding a permanent fort would in essence be founding a new settlement as support buildings and merchants set up shop in the area, as often happened in Roman times. In fact, many of the north european settlements in the game started out as Roman "forts" that grew into towns. The best example: London was founded on open ground as a fort to defend a crossing on the Thames.