When a fort is in mountains (there are four such spaces) does the fort get the benefit of the terrain if defending without other friendly units.
Rule 12.2.8 states (emphasis mine in bold):
Each player finds his Combat Strength on the appropriate Fire Table and shifts a number of columns depending upon the Terrain Effects of the defending space (terrain and trench effects are cumulative) to determine his Fire Column. Column shifts cannot cause the Fire Column to be off the Army/ Corps/Fort Tables; if the column shifts would take the Fire Column above the rightmost or below the leftmost column, any additional shifts are ignored.
These columns shifts apply to all units and forts in the space, except Forts cannot benefit from a Trench if no units are in the space:
15.1.6 Forts without friendly units in their space may never be the target of a Flank Attack or benefit from any trench in their space.