At Halagu's request, Ruknuddin sent his representatives with the Mongol envoys to all the castles in Rudhbar, instructing for their destruction. Some forty castles were thus demolished. Halagu proceeded to the foot of Alamut, whose Ismaili commander was Muqadinuddin. Leaving Balaghai behind to besiege Alamut with a large force, Halagu then set out for Lamasar. After a few days, the garrison of Alamut dismounted. Berthold Spuler writes in "The Muslim World" (London, 1969, 2nd vol., p. 18) that, "The fortress Alamut offered a desperate resistance to the onslaughts of the Central Asian hordes and only succumbed after a prolong siege." Towards the end of Zilkada, 654/December, 1256, all the persons in Alamut came down with all their goods and belongings and after three days, the Mongols climbed up to the castle and seized whatever those people had been unable to carry off. They also plundered freely whatever they found in the castle, and then set fire to its building and its library. Meanwhile, Ata Malik Juvaini, who had accompanied Halagu to the foot of Lamasar, had been granted permission to inspect the library. He saved a number of choice books, including some Ismaili works, as well as certain astronomical instruments, before consigning the library to flames. Thus, the accumulated literary treasure of about two centuries was consumed to ashes. Juvaini himself writes, "I burnt them all" (basukh tam). Edward G. Browne termed it, "world's renowned library." Arif Tamir writes in "Khams Rasail Ismailiyya" (Beirut, 1956, p. 195) that, "The Mongol destroyed the Ismaili library containing one and one half million volumes."
As for the Alamut, Juvaini writes, "It was a castle whereof the entries and exits, the ascents and approaches had been so strengthened by plastered walls and lead-covered ramparts that when it was being demolished, it was as though the iron struck its head on a stone, and it had nothing in its hand and yet resisted. And in the cavities of these rocks they had constructed several long, wide and tall gallaries and deep tanks, dispensing with the use of stone and mortar....And from the river, they had brought a conduit to the foot of the castle and from thence a conduit was cut in the rock half way round the castle and ocean-like tanks, also of rock, constructed beneath so that the water would be stored in them by its own impetus and was continually flowing on. Most of these stores of liquids and solids, which they had been laying down from the time of Hasan-i Sabbah, that is over a period of more than 170 years, showed no sign of destruction, and this they regarded as a result of Hasan's sanctity. (2nd vol., pp. 720-1) Juvaini goes on to tell how a large body of Mongol soldiers were employed in demolishing the castle: "Picks were of no use: they set fire to the buildings and then broke them up, and this occupied them for a long time." (Ibid.)
Meanwhile, at Lamasar, Halagu had failed to induce the surrender despite the services of Ruknuddin as intercessor. He left Dayir Buqa to beleaguer it with an army, but it did not surrender until 1258. He quitted Rudhbar on 13th Zilhaja, 655/January 4, 1257 and reached his encampment at Hamdan. On 22nd Zilhaja/January 13, Ruknuddin's family and servants were billeted in Qazwin, but he himself accompanied Halagu. From here, on Halagu's request, Ruknuddin sent his emissaries to the Ismaili castles in Syria, instructing them to guard the castels as subjects of Halagu until such time as he himself should arrive there.