The only things we can say with any real certainty about Hasan I Sabbah is that he

1: Was a Nizari Isma’ili Muslim and 2: Existed.

He is certainly cloaked in a huge amount of mystery and anti-Isma’ili polemicists, orientalists, and Crusaders returning to Europe with tales of the Holy Land have added a mystic dimension to his life.

To quickly answer your questions:

1. Most likely the book “Alamut” by Vladimir Bartol.
2. Since the mid to late 11th or early 12th Century CE.
3. Real.
4. It’s ruins still exist.
5. As it’s most commonly depicted none.
6. It’s almost certainly false.

Hasan I Sabbah was born into a Twelver Shia Muslim family around 1050 CE, before converting to Isma’ilism at age 17 and later moving to Cairo to better serve the Isma’ili Imam Caliph al-Zahir li-i’zaz Din Allah.

After about 3 years of study Sabbah left Cairo as a missionary (da’i) attempting to spread the Isma’ili faith further. Between 1081 and 1090 CE traveled Persia with his missionary work, as well as attempting to incite revolution against Seljuk control of the region. In 1090 CE Sabbah took control of Alamut, which would become the most famous of the Ismaili fortresses without bloodshed. This is most likely the “mountain palace” you are asking about. Sabbah fortified Alamut systematically and it would not fall again for another 166 years. He also irrigated and cultivated the Alamut valley which may well have been the inspiration for “the garden of paradise” that appears in later writings. The ruins of Alamut still exist today and can be toured should you wish to visit them in Iran.

In 1094 CE the Isma’ili faith was faced with arguably its most significant schism. Imam Caliph al-Mustansir Billah had died nominating his son Nizar to succeed him as Imam and Caliph. However his vizier al-Afdal Shahanshah conspired against his wishes to put al-Mustansir’s younger son al-Musta’li on the throne. This divided the Isma’ili population with some recognising al-Musta’li as the legitimate Caliph and Imam and others supporting Nizar’s claim to both. Sabbah was firmly in support of Nizar’s claim, a failed Nizari revolution in late 1094 resulted in Nizar’s execution and Sabbah’s imprisonment.

Here begins the more mythical tales about Sabbah, the most common being that a minaret fell on his cell. Leading to him being transferred as far away from Cairo as possible as the higher ups in the Fatimid Caliphate took this as a sign they weren’t safe from him. During this transfer Sabbah was placed onto a ship that capsized and Sabbah escaped into the mountains of Iran. Other than Sabbah’s retreat into the mountains, most of this is probably apocryphal.

Sabbah became Chief da’i, the highest position somebody who is not eligible to be the Imam can reach within the Isma’ili faith and returned to Alamut. Here he developed the Isma’ili doctrine of ta’lim emphasizing the authority of the Imam.

Edit as I realise I stopped talking about Sabbah at the most influential point of his life: From Alamut Sabbah would guide the Nizari from the remnants of a failed revolution to a serious player in the Muslim world. Knowing that they were relatively few in number Sabbah needed to focus on asymmetric warfare as the Nizari would not stand a chance in open battle against the Fatimids, the Seljuks or the Crusaders. He would train operatives in everything from language to philosophy in order that they would be able to infiltrate the courts of Sultans, Kings, and Caliphs. These operatives would become known as fidai (one who sacrifices) given their willingness to die in pursuit of their mission. The Nizari would assassinate key figures of the Seljuks Fatimids and Crusaaders to destabilise their power structures and prevent them from marching against them. They would also offer their services as a tool to curry favour with local rulers or threaten them to make them think twice about making an enemy that could strike them at any time.

This added to the cloak of mystery around Sabbah, who was accused of using black magic to control the fidai, or to cause the death of his rivals. If somebody died unexpectedly of disease or food poisoning Sabbah was a suspect. The Nizaris for their part allowed these myths to spread, making them seem more numerous or more dangerous than they likely were.

The Nizari would expand into Syria taking and fortifying easily defensible positions in the mountains. The Syrian Ismailis would act somewhat autonomously but were ultimately subject to Alamut, and the Imam that Sabbah claimed to speak on behalf of. Sabbah would rule in the name of the Ismaili Imam until his death in 1124 CE. The order of Assassins would continue after his death, and come under the direct control of the Nizari Ismaili Imam Hasan ala Dhikrihi’s Salam in 1162 CE. They would continue to feature prominently in Iran, Syria and the surrounding regions until Mongol invasions destroyed their State and forced them underground. End of edit.

By 1180 the Crusaders had some knowledge of Shi’ism given the history of Isma’ilis, Zaydis, and Twelvers in the region, although a lot of it was wildly incorrect. By the early 12th Century, the Crusaders and Nizaris were clashing frequently in Syria. Musta’li Nizari rivalry had intensified with then Fatimid Caliph al-Amir issuing an epistle calling the Nizari “Hashishiyya” for the first time. Despite its more obvious inference, the Nizaris most likely did not consume hashish as part of any ritual practice, and this is more likely a generic insult that stuck. It is now where the word “Assassin” derives from in English hence the Nizaris also being known as the order of Assassins or the Assassins Creed.

Benjamin of Tudela is most likely the first European to Chronicle the Nizaris in 1167 CE, although his account is very muddled. He does correctly the identify the link between the Persian and Syrian Nizaris.

The idea that the Nizaris were tempted with a promise of paradise that they experienced first on Earth was started by Burchard of Strassburg in 1175 CE. This includes children being taken to beautiful locations and taught to love, fear and obey their leader so that they will enter paradise when they die. Notably this does not include the use of hashish or beautiful women. These ideas were introduced by Marco Polo.

In summary Hasan Sabbah was a real person who’s legacy is tied up in so many myths that some of them have entered popular culture and even language itself. Unfortunately we’ll most likely never know all of the truth due to the Mongol attempts to wipe Isma’ilism off the face of the planet. Including the destruction of Isma’ili libraries and even Sabbah’s own biography.

Further reading:

Daftary F, The Assassin Lends: Myths of the Isma’ilis, IB Taurus, 1995.

Daftary F, The Isma'ilis Their History and Doctrines, Cambridge: Cambridge University Press, 1990.

Lewis B, The Assassins: A Radical Sect of Islam, Oxford University Press, 1967.