By itself, the remains of Rohtasgarh are less impressive compared to its location and geography. It should not be confused with Qila Rohtas which was named after Rohtasgarh by Sher Shah Suri, perhaps impressed by the defenses of Rohtasgarh-on-Son.
It is situated at the eastern end of the Kaimur range and overlooking the river Son. There is a core fort surrounded by a fortification that runs along the edge of the 1500-feet Rohtas Plateau making it one of the most impregnable forts in India.
The fortification runs along the edge of the plateau, while the plateau is connected to the rest of the Kaimur range by a thin strip of land on the western side.
In the terrain map above, zoom out to view a larger area and you’ll notice that the fort commands the plains and plateaus towards the east, a major reason why it was held by one power or the other from ancient times down to the few years following 1857, from Raja Harishchandra who is believed to have built it, to being coveted and captured by Sher Shah Suri, to being occupied by Kunwar Singh in the war of 1857, followed by the EEIC troops.
For more information, see the Bihar and Orissa Gazetteer on Google Books