The grandly proportioned St. Isaac's is the world's third-largest domed cathedral and the first monument you see of the city if you arrive by ship. Its architectural distinction is up for debate; some consider the massive design and highly ornate interior to be excessive, but others revel in its opulence. Tsar Alexander I commissioned the construction of the cathedral in 1818 to celebrate his victory over Napoléon, but it took more than 40 years to actually build it. The French architect Auguste Ricard de Montferrand devoted his life to the project, and died the year the cathedral was finally consecrated, in 1858.
The interior of the cathedral is lavishly decorated with malachite, lazulite, marble, and other stones and minerals. Gilding the dome required 220 pounds of gold. At one time a Foucault pendulum hung here to demonstrate the axial rotation of the earth, but it was removed in the late 20th century. After the Revolution of 1917 the cathedral was closed to worshippers, and
in 1931 was opened as a museum; services have since resumed. St. Isaac's was not altogether returned to the Orthodox Church, but Christmas and Easter are celebrated here (note that Orthodox holidays follow the Julian calendar and fall about 13 days after their Western equivalents).
When the city was blockaded during World War II, the gilded dome was painted black to avoid its being targeted by enemy fire. The cathedral nevertheless suffered heavy damage, as bullet holes on the columns on the south side attest. The outer colonnade beneath the dome affords an excellent view of the city, especially at twilight and during the the famous White Nights.
To one side of the cathedral, where the prospekt meets Konnogvardeisky bulvar, is the early-19th-century Konnogvardeisky Manège, gracefully designed by Giacomo Quarenghi and decorated with marble statues of the mythological twins Castor and Pollux. This former barracks of the Imperial horse guards is used as an art exhibition hall.