The Hermitage is one of the world's oldest art galleries and largest museums. It occupies six historic buildings along the Palace Embankment.
In the 20th century, during Soviet times, the square was a symbol of revolutionary passion. The classic film October shows angry sailors attacking the Winter Palace across the square in 1917, althogh in fact it never happened. The director, Sergey Eisenstein, invented it; art making history.
Behind it, the Hermitage has 3 million works of art on display or in storage. A cruise visitor could easily spend every moment ashore within its walls and still see only a fraction. To avoid exhaustion it is best to look at the Hermitage web site in advance and decide what collections interest you the most.
As the 112 solid granite columns suggest, it was designed by the same architect who created the Alexander Column in Palace Square, French-born August de Montferrand. St Isaac's took 40 years to build.
The exterior looks severe. The interior is sumptuous. After the Russian revolution, it was turned into a Museum of Atheism. Now one of its chapels is used for worship again, but even agnostics will appreciate the spectacular view from its observation platform.
Students of politics will enjoy the inscription "Catherine II to Peter I", intended to underline Catherine's status as Peter's successor. In fact she was a German princess who married into the Russian royal family and took over in a palace coup.
The statue stands 6 metres (20 feet) tall and shows Peter on his horse. It got its nickname fifty years after it was completed, from a poem called "the Bronze Horseman", by Alexander Pushkin. It describes one of the many times that the River Neva has overflowed its banks.
In the poem, a flood victim curses Tsar Peter for founding St Petersburg in such a dangerous place. The statue then comes to life and chases him through the city to teach him some respect. There is a modern lesson here, not necessarily related to flooding.