Possibly one of the most impressive buildings I have ever seen, At 600 meters (Almost 2,000 Ft and damn near half a mile), the General Staff Building may be the longest building in Europe. It was commissioned in 1819 to house the Russian army's general staff. The structure, designed by Karl Rossi, is actually a facade that enclosed two existing buildings. A triumphal arch separates the two wings. While not yet open to the public, this building is supposed to become an integral part of the Hermitage complex.

A good source of architectural info on the building is available here for those who are into such things.