Before he designed St Paul's Christopher Wren wrote to 'Turkey' North, then the most important English merchant in Istanbul, to ask how the Ottomans built their domes. North wrote back that lead should never be laid on wood but on 'that earth from which it came'. Like all the work of Sinan, the supreme Ottoman architect, the domes of the Atik Valide Sultan Mosque above Üsküdar conformed to this rule. Once in a while, however, the lead must be relaid to withstand the ravishes of hail, crow and gull.

Originally the lead came from the mines of Anatolia. Lead ingots were melted down into moulds, much as children used to melt lead over an open fire to pour into moulds for toy soldiers. The somewhat larger moulds of the Ottomans produced sheets of lead a metre wide. These can be reused after melting down, but their bed has to be completely renewed in accordance with the ancient formula: a layer of horusan – brick dust mixed with mud (obviating the Byzantines' use of blood) – and a paste of earth and straw.

The craftsman's skill lies in the marriage of the sheets of lead. The edges of neighbouring sheets are turned up, with one side higher than the other; the top side is then bent over its companion and the two are rolled to form a tight question mark. This forms the ribs which radiate from the crown of Ottoman domes and are so important aesthetically. 

A steady hand and a skilled eye are essential to taper each sheet of lead as the segments of the dome gently narrow towards the crown, which will be topped with a gilded alem in the shape of a crescent moon. The hoods of the minarets are too steep to hold horasan, so there the lead is bedded on felt. When lead is laid it shines like silver, once its bedfellow in the mine, but it soon oxidises, turning black before it fades to a soft grey, giving the illusion of moonlit lawns spreading over the vales and hillocks of the masonry.

The Ottomans took the hemispherical dome and made it their own, rejecting elliptical and onion forms. Hemispheres floated from Buda to Arabia, speaking a dialect and subject people could understand. The greatest Ottoman achievement is the dome of the Selimiye Mosque at Edirne, where autocracy turns to poetry.

The Atik Valide Mosque in Üsküdar may have a dome of lesser magnitude but its twin minarets mark it out as a royal mosque – a sign of respect on the part of Murat III for his mother, Nurbanu, after whom the mosque is named.

Nurbanu (literally, Princess of Light) was born Cecilia, the illegitimate child of a prominent Venetian. She was captured by the legendary Hayrettin Pasha – Barbarossa to the West – as she made her way back to Venice across the Aegean, where her father was governor of one of the islands. Noting her exceptional beauty and intelligence, Barbarossa saw to it that she reached the imperial court in Istanbul. Once there she went on to marry Selim II, while he was still the heir to the sultanate.

Her early married life was spent happily in Manisa, on the Aegean, where Selim was governor. She loved its palace, whose terraced gardens supplied daffodils and roses to the Topkapı, and it was there that she gave birth to her son Murat. But the death of Süleyman the Magnificent put an end to pleasure. She was soon to become the power behind the throne, first as wife to Selim II, then as Valide Sultan, or Queen Mother, in the reign of Murat III. Her influence is likened to that of Catherine de Medici in France.

When Murat succeeded to the throne, he gave his patronage to his mother's charitable work. She established her foundation in Üsküdar, with its sweeping garden filled with trees and birds, its grand imaret, or charity kitchen, now a prison, and the spacious asylum where the deranged were nursed by dervishes, now a refuge for orphans. The school, or medrese, follows the contours of the hill below the mosque. The library spans a lane, in imitation of the Bridge of Sighs.

And all the time Nurbanu corresponded with Elizabeth I and the Doge of Venice, from both of whom she solicited gifts. When she was dying slowly and painfully, her last request was for cushions. They arrived too late. Her heart, she wrote to the Doge, belonged to Venice, although fate had had other things in mind.