Merv reached its heyday in the XI-XII century, when it became the largest city and then the capital of the Turkmen state under the dynasty of the Grand Seljuks. The city center shifts to the territory of Sultan-kala with the citadel Shahriyar-Ark. East of Gyaur-Kala, a fortified camp of the Seljuk troops with an area of 120 hectares is built, which today is called Shaim-Kala. New districts bordering the Sultan-Kala from north and south are also surrounded by a fortified wall. Thus, the Seljuk Merv in the plan extends along the north-south axis, along which the whole city is cut through by the Majan channel. Along this, the main street of the city extends with the entrance gates at the ends. In the middle of the western and eastern city walls there was also an entrance gate, which was connected to the city by another connecting road. At the crossing of these streets, in the heart of Sultan Kala, there was a walled square cloister with a Seljuk palace, a mosque in the dome of the cathedral and the mausoleum of Sultan Sanjar.
Besides trade, science and art flourish in the city. The beauty and wealth of Merv and the richness of its bazaars throughout the Muslim East were legends, and its famous libraries attracted renowned scientists of the time. After the death of the last Seljuk ruler, Sultan Sanjar, and the collapse of the Great Seljuk Empire, Merv became part of Khorezm. At the end of the XII – beginning of the XIII century, under Khorezmshakhs, after some decline, Merv becomes again a crowded city with developed economy and prosperous culture. The great geographer Yaqut al-Hamawi, who left Merv in 1219, wrote: “…When I left Merv, it was in its most prosperous state If it had not been for what happened when the Tartars invaded and destroyed these lands, I would not have let Merv die because of the politeness, warmth and friendliness of its people and the many books on basic sciences … At the time of his departure, there were ten bookshops – so-called Waqfs…”
According to the available information, when the 80000 strong Mongolian army came to Merv in 1221, the son of Genghis Khan Tolui Khan is said to have spent “six days looking at the ramparts, walls, moats and minarets of the city and doubting the success of such Merv fortresses. However, intrigues and disputes among the defenders did not allow a decent defense of the city to be organized, which was cleverly used by the enemy. The consequences of the Mongolian pogrom were terrifying. According to the medieval historian Juweini, the number of killed inhabitants of Merv exceeded 1 million 300 thousand people. After this catastrophe, Merv was unable to recover for many decades.
In the XV century, the new governor of Khorasan, son of Amir Timur Shahruch, tries to revive a once great city: For this purpose, a new city fortress Abdullahan-Kala is built and populated south of Sultan-Kala, and west of it another fortress is built by another Timurid Mirza Sanjar, who later took the name of the XVIII century Turkmen ruler of the Bairamali-Khan.
Mirza Sanjar is delighted that Mirza Sanjar continues the reconstruction of Merv, the famous poet Alisher Navoi dedicated poems to this occasion. However, no one succeeded in reclaiming the glory of Merv from the time of Seljuk Sanjar. Throughout its history, the Timurid Merv has been the target of constant demands from Bukhara, Khiva and Persia. They unsuccessfully defended their claims to the city and the native Turkmen, who completely took the city at the beginning of XIX century, fortified the walls of Bairamalikhan-Kala. Soon, however, due to the destruction of the Sultanbent Dam, the city, which had to be abandoned by its inhabitants, moved 30 km to the west, where the modern city of Mary is located today.
Merv, which was left in ruins, is now the largest archaeological reserve in Central Asia. It consists of several contiguous settlements from different eras, which contain information on the urban planning and material culture of the famous city from the middle of the first millennium BC to the late Middle Ages. These are the already mentioned sites of Erk-Kala, Gyaur-Kala, Sultan-Kala, Abdullahan-Kala and Bairamalikhan-Kala. In and around these sites a number of monuments from the Middle Ages have been preserved.