Haghia Sophia
Haghia Sophia, known to the Turks as Aya Sofya and literally translated as ‘Church of Divine Wisdom', is generally considered the finest extant example of Byzantine architecture in the world, although many of its interior columns were pilfered from pre-Christian temples. Consecrated in 537AD, its vast dome rises to 56m (183ft), designed to appear suspended in space and thus representative of heaven. Two other churches were built on the spot but were destroyed and Haghia Sophia remains a testament to the vision of Emperor Justinian and the sixth-century Greek mathematicians who were able to apply the latest developments in geometry to architecture. So impressed was Mehmet the Conqueror that when he took the city in 1453, he dedicated it as a mosque to Allah and it remained so until declared a museum at the founding of the secular Turkish Republic. Fortunately, many of the late Byzantine mosaics remain, as do the huge circular shields from the Ottoman period containing calligraphy of Koranic verses. The museum is now divided into two parts – the church and the mosaic gallery – each requiring a different ticket.

Topkapi Palace
Mehmet the Conqueror originally built Topkapi Palace as a summer residence and the seat of government over the acropolis of pre-Christian Byzantium. It was his great grandson, Süleyman the Magnificent, who decided to consolidate home, harem, state administration and military personnel at Topkapi in the 16th century. In keeping with the Ottomans' nomadic Central Asian origins, the complex looks more like a walled city of tent-like pavilions than a royal fortress. At its height, the palace spread over the whole of Seraglio Point to the Golden Horn and the Sea of Marmara and had 3000 residents.
Sultans abandoned it for the more European Dolmabahçe Palace, in 1855. However, the sumptuous jewels of the original treasury (including the Topkapi Dagger, the 86-carat Spoonmaker's Diamond and gold-plated throne of Murat III), the armoury, the display of silk ceremonial robes, the Chinese and Iznik ceramics, holy relics and the collection of manuscripts and miniatures, all convey the extent of the Ottoman lifestyle at its peak. The harem, which once housed up to 300 concubines, is only open to guided tours and requires a separate ticket, available for purchase inside the complex. However, it is not at all glamorous and somewhat similar to a lavishly decorated women's prison. Topkapi requires at least half a day for a full visit

Sultanahmet Camii (Blue Mosque)
The Blue Mosque was built during the reign of the young Sultan Ahmet (1603-1617), as Islam's answer to Haghia Sophia. To this day, the magnificent mosque is the symbol and centre of religious demonstrations. It takes the name ‘blue' from the quantity of Iznik tiles that dominate the interior, giving a cooling effect, as well as the blue light that shines through more than 250 windows. From the small dome at the entrance gate to the vast central dome – some 43m (14ft) high and 23.5m (77ft) in diameter – architect Mehmet Aga's intention was to lift all eyes heavenward, even from a distance. The Imperial Loge is covered with fine latticework and the mihrab (prayer niche) contains a piece of sacred black stone from the Ka'aba in Mecca. During the summer months, there is a Son et Lumière show at dusk. The Imperial Pavilion also contains a state-run carpet museum with Usak, Bergama and Konya samples, dating from the 16th to the 19th centuries.

Great Palace Mosaic Museum
The small Mosaic Museum, tucked behind the Blue Mosque amid an alleyway full of tourist shops, contains all that is left of the magnificent flooring of Bucoleon Palace. This was the Great Palace of the Byzantine emperors, which once covered the area from the Hippodrome to Haghia Sophia and down to the harbour. Discovered in the 1950s, these mosaics depict mythological and hunting scenes, including that of two men subduing a tiger. They are thought to date from the reign of Justinian (527-565AD) or even earlier and to have been part of the walkway between the royal apartments and the imperial seats (kathisma) at the Hippodrome.

Yerebatan Sarayi (Basilica Cistern)
Sometimes referred to as the Sunken Palace, the Basilica Cistern was actually the reservoir for the prodigious quantity of water required for the Byzantine Great Palace and it is thought to date back to the reign of Justinian in AD532. It is a huge structure, measuring 140m (460ft) long and 70m (230ft) across and it still contains a few feet of water, over which wooden walkways have been constructed for visitors. The atmosphere is enhanced by low, eerie lighting and classical music. Many of the 336 columns supporting the cathedral-like ceiling have been recycled from pre-Christian temples – such as the Medusa heads that are used as column bases, which were pilfered from the Temple of Apollo in Didyma (Didim) and can be seen near the exit. Various theories, legends and superstitions surround these eerie gorgon faces from ancient Greek mythology. The cistern was used as a film set for the James Bond film, From Russia With Love (1963) and also functions as an atmospheric gallery space during the International Istanbul Arts Biennial (September-October 2003 and 2005).

Dolmabahçe Palace
Dolmabahçe Palace was built in the mid-19th century, as the Ottoman Empire was falling into decline and most of the economy and development of the country was in the hands of Europeans. The Sultans wished to appear more Western and the new palace was intended to put on a brave face for the world. Instead it nearly bankrupted the state treasury with its kitsch extravagance and much of it had to be financed by foreign loans. Today, it is in poor repair, which partially explains the high entrance fee. Highlights include the crystal staircase, the royal bathroom – made of silver and rare Egyptian alabaster – and the chandelier in the ceremonial room, rumoured to be the heaviest in the world. The palace is also full of gifts from foreign rulers. There are two separate guided tours – one to the ceremonial chambers and another through the harem and concubine's quarters. Entrance to the palace is with a guided tour only.

Beylerbeyi Palace
Beylerbeyi Palace is located in the Anatolian side of the Boshporus, in the province having the same name with the palace. The palace, making up a complex with the palace in the yard and the surrounding buildings, was commissioned by Sultan Abdülaziz to architect brothers Sarkis and Agop Balyan in 1864. The palace comprises of the Beylerbeyi Palace as the main structure, sea mansions one of which is women hall and the other is progression hall located in the sea front walls of the palace, Marble Mansion, Yellow Palace and Hasahir in the backyard. While the sea mansions and Beylerbeyi Palace were commisioned by Sultan Abdülaziz, the other buildings are known to be a part of the palace once located on this spot. Beylerbeyi Palace, the main unit of the palace complex, is a two-storey stone building built above a high cellar. The length of the palace which is built in parallel to the Boshporus is 65 meters. There are 6 saloons and 24 rooms in the palace which has staircase access from three sides. Especially Fountain Saloon and Blue Saloon which has its name from the color of its columns on the upper floor are the most impressive places of the palace. Also its garden arranged in sets is another feature of the palace.