The dome of St. Isaac’s Cathedral dominates the skyline of St. Petersburg and its gilded cupola can be seen glistening from all over the city. You can climb up the 300 or so steps to the observation walkway at the base of the cathedral’s dome and enjoy the breathtaking views over the city.

Sources, related links:
Virtual Tour - Saint Isaac´s Cathedral
Wikipedia - Saint Isaac´s Cathedral
Saint Isaac´s Cathedral - official web site