is encircled by a walkway accessible to tourists. 24 statues stand on the roof, and another 24 on top of the rotunda. Dome The cathedral's main dome rises ; its exterior is gilded. The dome is decorated with twelve statues of angels by Josef Hermann. These angels were likely the first large sculptures produced by the then novel process of electrotyping, which was an alternative to traditional bronze casting of sculptures. Montferrand's design of the dome is based on a supporting cast iron structure. It was the third historical instance of cast iron cupola after the Leaning Tower of Nevyansk (1732) and Mainz Cathedral (1826). With an internal height of 69 m (from the floor to the level of the oculus), it ranks among the tallest domes in the world. Interior The cathedral's bronze doors, covered in reliefs by Ivan Vitali, are patterned after the celebrated doors of the Battistero di San Giovanni in Florence, designed by Lorenzo Ghiberti. Suspended underneath the peak of the dome is a sculpted white dove representing the Holy Spirit. Internal features such as columns, pilasters, floor, and statue of Montferrand are composed of multicolored granites and marbles gathered from all parts of Russia. The iconostasis is framed by eight columns of semiprecious stone: six of malachite and two smaller ones of lazurite. The four pediments are also richly sculpted. The interior was originally decorated with scores of paintings by Karl Bryullov and other great Russian masters of the day. When these paintings began to deteriorate due to the cold, damp conditions inside the cathedral, Montferrand ordered them to be painstakingly reproduced as mosaics, a technique introduced in Russia by Mikhail Lomonosov. This work was never completed. Technologies William Handyside and other engineers used a number of technological innovations in the construction of the building. The portico columns were raised with the use of large wooden frameworks before the walls were erected. The building rests on 10,000 tree trunks that were sunk by a large number of workers into the marshy banks upon which the cathedral is situated. The dome was gilded by a technique similar to spraypainting; the solution used included toxic mercury, the vapors of which caused the deaths of sixty workers. The dozen gilded statues of angels, each six metres tall, facing each other across the interior of the rotunda, were constructed using galvanoplastic technology, making them only millimeters thick and very