Their impressive minarets and domes dominate the skyline of Istanbul and no tour to the ancient Turkish city is complete without a visit to its two most important landmarks -- the Hagia Sophia and the Blue Mosque.

The Hagia Sophia, built in the 6th century, functioned as a church for more than 900 years, till the Ottoman Turks converted it into a mosque in the 15th century and it remained so for almost 500 years. It was in 1935 that the farsighted first Turkish president and founder of the Turkish Republic, Mustafa Kemal Atatordered the Hagia Sophia to be converted into a museum.

After the building was transformed into a museum, prayers were no longer allowed inside. The layers of plaster covering the mosaic pictures of Christ and Virgin Mary on the ceilings of the gallery, applied during the Ottoman rule, were removed in many places to show the pictures and the massive building was thrown open to the public.

The cathedral, built by Emperor Justinian I, is one of the finest examples of Byzantine architecture. In 1453, the Ottoman Turks conquered Constantinople, what is modern-day Istanbul, and converted it into a mosque, calling it the Ayasofya.

“The Ottoman rulers did not demolish anything inside the structure, they had the greatest respect for the Christian faith. They only carefully plastered over the Christian emblems inside the Hagia Sophia,” tour guide Derya Kutukcu told this visiting IANS correspondent.

The Ottomans added the three important symbols of Islam in the building -- the mihrab, or a niche in the wall pointing to the direction of Mecca; the mimbar or pulpit for the cleric; and the minaret for the azan, as well as huge discs with inscriptions in praise of Allah on it.

The Turks after taking over the building, added huge buttresses to it to strengthen the structure and the minarets to give it the element of a mosque.

“Look carefully at the dome. There is no steel holding the stone pieces together,” Derya said. One cannot help but marvel at the feat of the engineers who created the masterpiece so many hundreds of years ago.

The dome, if you stare at it from the groundfloor, looks perfectly formed but it is not, said Derya. It is 31.7 metres in diameter on one side and 31.4 metres on the other. “It is not a perfect circle,” she said.

The Blue Mosque, Istanbul’s famous landmark, is just next door to the Hagia Sophia.

Built by Ottoman emperor Sultan Ahmet I during 1609-1616, it is the first mosque to have six minarets. The Blue Mosque gets its name from the blue Iznik tiles adorning its interior walls.

The mosque, which is a functional mosque, is open to tourists, barring an area in the central hall which is kept for offering prayers.

Since footwear is not allowed inside, the authorities have provided plastic packets for tourists to wrap their footwear in while walking inside the main hall.

The huge central dome has four quarter domes on four sides. It has two floors, with the second floor for the women to sit and pray.

Since many of the precious Iznik tiles, which are made of quartz, were stolen in the early years, guards stand all along the main hall and don’t allow anyone to go near the walls.

“Please, please...,” they politely say and wave their hands to indicate to tourists to keep away from the wall.

There are 21,043 pieces of square Iznik tiles, each side measuring 23.5 cm, plastered on the walls, according to Derya.

The mosque also has a madrassa, which functioned like a university for the Ottomans, and a fountain for ablutions before the prayers.

Clerics with very melodious voices sing the azan prayers, which blare out from the speakers on the minarets.

The city boasts of another beautiful mosque, the 16th century Suleiman Mosque built by Ottoman rule Suleiman the Magnificent. It is located near Istanbul University and the city’s well-known Grand Bazar.