emergence of the Parthians and Sassanids new forms appeared. Parthian innovations fully flowered during the Sassanid period with massive barrel-vaulted chambers, solid masonry domes and tall columns. This influence was to remain for years to come. For example, the roundness of the city of Baghdad in the Abbasid era, points to its Persian precedents, such as Firouzabad in Fars. Al-Mansur hired two designers to plan the city's design: Naubakht, a former Persian Zoroastrian who also determined that the date of the foundation of the city should be astrologically significant, and Mashallah ibn Athari, a former Jew from Khorasan. The ruins of Persepolis, Ctesiphon, Sialk, Pasargadae, Firouzabad, and Arg-é Bam give us a distant glimpse of what contributions Persians made to the art of building. The imposing Sassanid castle built at Derbent, Dagestan (now a part of Russia) is one of the most extant and living examples of splendid Sassanid Iranian architecture. Since 2003, the Sassanid castle has been listed on Russia's UNESCO World Heritage list. Islamic architecture of Persia The fall of the Sassanian dynasty to the invading Muslim Arabs led to the adaptation of Persian architectural forms for Islamic religious buildings in Iran. Arts such as calligraphy, stucco work, mirror work and mosaics became closely tied with the architecture of mosques in Persia (Iran). An example is the round-domed rooftops which originate in the Parthian (Ashkanid) dynasty of Iran. Archaeological excavations have provided extensive evidence supporting the impact of Sassanid architecture on the architecture of the Islamic world at large. The Razi style (شیوه معماری رازی) is a term for the used between the 11th century and the Mongol conquest of Iran, reflecting influences from Samanid, Ghaznavid, and Seljuk architecture. Examples of the style include the Tomb of Isma'il of Samanid, Gonbad-e Qabus, the older parts of the Jameh Mosque of Isfahan and the Kharaqan towers. Many experts believe the period of Persian architecture from the 15th through 17th centuries CE to be the pinnacle of the post-Islamic era. Various structures such as mosques, mausoleums, bazaars, bridges and palaces have survived from this period. Safavid Isfahan tried to achieve grandeur in scale (Isfahan's Naghsh-i Jahan Square is the sixth largest square worldwide), by constructing tall buildings with vast inner spaces. However, the quality of ornaments was less compared to those of the 14th and 15th centuries. Another aspect of this architecture was the harmony with the people, their environment,