The game will be able to place a road between two neighbouring cities which share a region boundary so long as the region boundary isn't completely blocked by either sea, mountains (high or low), or dense forest, or a combination of those impassible ground types.

If it can place a road it will try to use the 'fastest' path which would mean it prefers to go over the fertile grass groundtypes. The others hills, swamp and sparse forest might be able to deflect the road route a bit but it won't take a very much longer route just to avoid them . (I think long while since I messed about with that)