What I can say is that this is castle country on a grand scale. The Alborz are a wall of steep, folded mountains that rise above the Caspian coast to the peak of Damavand and beyond, and every ridge worth defending seems to have a ruin on it, mud-brick and stone ramparts clinging to the crags.
The most famous is Alamut itself, the stronghold of Hassan-i Sabbah and the Nizari Ismailis, whom crusader writers called the Assassins. They held a string of mountain castles from the late eleventh century until the Mongols destroyed them in 1256. The story of their young recruits and the garden of paradise is mostly legend, but the real castles are dramatic enough, perched on rock spurs with a single path up and a single cistern cut into stone. Smaller forts like this one belong to the same landscape of passes, valleys and small walled villages.
The riding here is serious and beautiful. Roads climb from the plain at Qazvin through foothills of orchards and walnut trees, then wind in tight bends along river gorges, with sheer rock on one side and a drop on the other. The Chalus road, over the high passes to the Caspian, is among the great mountain roads of the region, but it is also heavily used by lorries and holiday traffic, and snow closes the upper sections for months in winter. Surfaces can be broken, landslides leave gravel on the road after rain, and mist comes down fast.
Villages in the valleys have little more than a shop and a tea house, so fill the tank in Qazvin or Rudbar before heading up. Come between May and October, when the passes are open and the walnut orchards are green, and carry a layer for the altitude. Check current travel advice and local road conditions, and leave plenty of daylight for the return.