Due to some underlying factors that may benefit each player in the game, it is important to distinguish between direct and indirect adjacency of Terrain spaces.
Direct adjacency
Terrain spaces and Structures are considered directly adjacent to one another if they share a hexagon edge. Additionally, Terrain spaces and Structures separated by a River but connected via a Bridge are also considered directly adjacent to one another.
Indirect adjacency
Terrain spaces and Structures are considered indirectly adjacent to one another if they are (not directly adjacent to one another, but) separated by one or more River spaces and your Shipping Skill Level is high enough to cover the distance. (Terrain spaces and Structures that Dwarves can reach via Tunneling and Fakirs via Carpet Flight are also considered indirectly adjacent to one another).