Recommended to set graphics - custom - terrain detail: medium or high to see the roads.
Read the mission briefing before you load the map.
Extract it to C:\Users\username\Documents\Stronghold 3\Maps
and select it in game from Single player - Game Modes - User Maps
At the start of the map you will hear "the lord has been killed" this is because you have 2 keeps and one of them(the on on the left) is attacked and the lord there dies. This does not affect your gameplay, it is how this map works, and you need to take that keep and defend it from the final invasion.