North of Qazvin, the Alborz rises in folded, ochre and green ridges, and somewhere in those ridges lies Širkūh, one of the mountain strongholds of the Nizari Ismailis, the sect that held the valleys of Alamut and Rudbar from the late eleventh century until the Mongols came. Their founder in Persia, Hasan-i Sabbah, took the castle of Alamut in 1090, and from it a network of fortresses grew up across these mountains. The rides that pass here link several of them, Qosṭinlār and Navizaršāh among them.
What survives at places like Širkūh is usually fragmentary: rock-cut cisterns, stubs of wall, terraces on an outcrop that commands a valley. Do not expect a restored castle. What you get instead is the logic of the place, a summit with water, food stores and a view of every approach, which allowed small garrisons to hold out against much larger armies. In 1256 Hulagu's Mongols forced the surrender of the Ismaili leader and demolished the castles one by one, and the legends of the so-called Assassins, much embroidered by European writers, did the rest.
The road from Qazvin north into the Alamut valley is one of the great rides of northern Iran. It climbs over a high pass with long views back across the plain, then drops into a steep valley of orchards, rice terraces and small villages, with the Alamut river cutting through. The tarmac is generally decent but narrow, with tight hairpins, rockfall after rain and the occasional livestock crossing. Snow can close the high section in winter, and spring brings meltwater and landslides. Late spring and autumn are ideal, with clear air and poplars turning gold.
There is no town of any size close to Širkūh itself, so treat Qazvin, Ābyek or Alvand, all around forty kilometres away, as your real supply points. Fill the tank and carry water before you head into the mountains. Qazvin itself is worth a night: a former Safavid capital with a fine old bazaar, mosques and caravanserais, and a lively food scene with local sweets and rice dishes.
As in the rest of Iran, foreign riders need a carnet, a valid visa and an up to date read on local rules. Village hospitality in the Alamut area is generous, and a stop for tea in a roadside teahouse with a view of the valley tends to stretch longer than you planned.