It is extremely difficult to find unused soil in Istanbul today. One can excavate as deep as 8-10m without encountering unused soil for a number of different reasons. The soil covering Istanbul today is full of rubble from various different periods. During the construction of the Botanical Institute, excavations were made to a depth of 15m and rubble was still encountered even at that depth. The depth of the soil layer in Istanbul has increased by between 2 and 15m according to place. The same thing could be said about its coastal strip as well. These coasts, which at one time featured a large number of inlets and capes, have disappeared and the city's old silhouette has been entirely erased. This is a result of deposited rubble, silting and the construction of quays and wharves. Thus, in the stretch between Sarayburnu and Eminönü, referred to as Sirkeci, the site of the old Byzantine harbor of Neorion has been entirely erased by the new wharf and this section of the coast is now straight. There is no trace of the old Byzantine harbors that were known to exist on the Marmara coast. The former Elefterios harbor silted up in the course of time, becoming an area in which vegetables were cultivated and is now the district known as Langa. It is known that the old lulianos harbor was renamed Kadırgalimanı during the Turkish period and was used as such for a number of years. Thus, in an engraving of İstanbul, the oldest version of which was made in circa 1515 and the original of which dates from the reign of Mehmet the Conqueror, it can be seen that a galleon is being built in the shipyard in this harbor.
The change in the coastline that began to take place in the course of the large amount of building that went on in the l9th century with the dumping in the sea of earth from foundation excavations has continued to this day, with the dumping of earth from the foundation pits of the University, the Central Law Courts and the Municipality building. This has happened to such an extent that new capes have appeared which completely changed the shape of the city. It is a fact that Istanbul has been built on seven hills. Six of these hills extend in a chain parallel to the Golden Horn as a last extension of the Strandja Mountains. The seventh of these hills stands alone, in the Cerrahpaşa district of the city on the lower side of the Bayrampaşa Creek. The locations of these hills, which decrease in height as one approaches Sarayburnu, could be summarised as follows:
1. The highest hill, which is on the inside of the city walls at Edirnekapı, where the Mihrimah Mosque stands,
2. The hill on which the Sultan Selim Mosque stands,
3. The hill on which the Fatih Mosque stands,
4. The hill on which the Süleymaniye and Beyazid mosques stand,
5. The hill on which the district of Çemberlitaş stands,
6. The hill on which Ayasofya and the Topkapı Palace stand,
7. The hill in the Cerrahpaşa-Altınmermer district of the city.
Of these, the hill mentioned in bullet point 2 has fairly steep sides, and there is quite a deep valley between hill number 3 and hill number 4. While Atatürk Boulevard was being built here in 1940, it was observed that the altitude of the ground was quite high and that it no longer resembled a valley. In order to transfer the water supplied to the city in Roman times from hill number 3 to hill number 4 it was necessary to build the Bozdoğan Aqueduct, which is still in existence.
Because of the hilly nature of the city, certain features have emerged that make Istanbul what it is. In the distant past, in order to carry out large-scale constructions it was considered necessary to build a number of terrace (retaining) walls, large underground store-houses were also constructed. Thus, it is possible to see the terrace (retaining) of walls in old parts of the city. On the other hand, main roads and streets tended to be curved. The building of streets on a grid system, or attempts to change the curves in the roads have resulted in problems such as those encountered in Beyazid Square, Ankara Avenue, the small square in front of the Governor's Office or Fevzipaşa Avenue in Fatih, all of which have proved very expensive in terms of solutions.
The Galata and Beyoğlu side of the city on the other hand, is a conical hill rising from the coastline itself. The Beyoğlu of today has been developed on the high land behind the conical hill of Galata. Due to the fact that this hill has very steep sides, the streets in this area have for many years been built in the form of steps. Old photographs provide evidence that up to the second half of the last century the Galata-Tophane coastline consisted of sand and gravel; towards the end of the l9th century a wharf was constructed by a foreign company, which transformed it to its present state. Üsküdar, the third part of the city, has come to look like a town covering a number of hillsides that rise from the coast itself. The districts lying between Şemsipaşa and Ayazma, on the other hand, resemble a high cape or promontory. In front of this promontory there is a small island very little higher than the level of the sea. It is known that in the very distant past there was a sort of toll station on this islet where tolls were collected from ships passing through the Bosphorus. In the Turkish period the same building was used as the foundation for a lighthouse. It is now known as Kız Kulesi, or the Maiden's Tower.
The highest peak in the İstanbul vicinity is the 288m Çamlıca Hill. It commands a magnificent view that embraces the mouth of the Bosphorus, old İstanbul, Galata and Beyoğlu. This peak, which was probably at one time covered with pines, is the first place one should make for if one wishes to understand the magnificence and beauty of the place where the city was founded. In this respect Çamlıca is one of the most important tourist spots in the city. The 200m Yuşa Hill, which also overlooks the Bosphorus, commands an equally magnificent view.