Fort Boyard, France - The storms that raged off the coast of France meant that they were working in perpetually dangerous conditions, and by the time it was built, it was already obsolete. New weapons technologies meant that the fort had outlived its usefulness before it was ever completed in 1859.
This includes Sealand: The Maunsell Forts were small fortified towers built in the Thames and Mersey estuaries during the Second World War to help defend the Unite... http://en.wikipedia.org/wiki/Maunsell_Forts
Fort Boyard, France - The technology of the time wasn’t quite up to the standards that his plans required, though, and it wasn’t actually begun until another of history’s infamous rulers came to power – Napoleon. Construction on the fort finally began in the early 1800s, but it ended up taking more than 50 years for construction crews to finish.