Very Briefly, The sky line that is Istanbul is the creation of Mimar Sinan, Sinan the Master Builder.

Sinan was the chief engineer of the Empire, and in charge of all public buildings, engineering projects like water works and bridges, military construction, and so forth. Perhaps his greatest contribution perhaps to world history is his rescuing and his restoration of Hagia Sophia (Ayasofya), the most famous of churches in European history, and the most important architecturally. Hagia Sophia represented Byzantine Rome, and it was the cathedral of the emperor. When Constantinople was conquered by Sultan Mehmet, Sultan Mohammed II, the Conqueror, it became the seat of the Islamic world's political and theological center. Mimar Sinan was able to return it to its former glory by making major and substantial repairs. In the process, Hagia Sophya has also become a world standard of architectural excellence. No one today could not have imagined this glorious and divine structure if Sinan had been anyone else. He has preserved it and made it possible to experience what it has been for people for tens of centuries.

The rest of his life Sinan strove to achieve himself a work which matched or surpassed it as a place to worship and Sinan's highest achievement and greatest masterpiece is in Edirne, the Selimiye Complex, including its mosque and school and all its support and maintenance structures and meeting facilities. Selimiye is one of the greatest masterpieces of architecture of all times, but any mosque or medressa or any of his buildings, damns, aquaducts, bridges, and places of study and worship in the Istanbul area are masterpieces and treasure of incomparable beauty and deepest spirituality. The least of his mosques take you to a space where there is no seperation between this world and the divine. Nothing he built is less than a complete masterpiece. 

 His greatest achievement is visible in his least work, complete unity of detail and design and structure, profound sense of humor, love of solid materials and transparent space, deep and immeasurable respect for the dignity of the individual, and inner spiritual peace. 

One of his Mosque which is very near to Sultanahmet Square is Sokollu Mehmet Pasha Mosque. The mosque Built in the Name of Sokollu Mehmet Pasha who was Grand Vezir of Magnificient Süleyman. The mosque itself is a full living Medrese. Not just a place to pray but also a school for young kids. Most of the kids actullay live and get education. There are financial sources ( like buildings to rent ) to keep Mosque alive for more than 500 years. For muslims there is a sacred Black Stone in Kaaba and 3 pieces of this stone has been located in this Mosque. One of them on the entrance door hight in the middle seond one is on the top of the Mihrab and the third one is on the entrance of Mimbar ( Pulpit ) It is actually on a distance you may touch.