LONDON -- The special relationship between the U.S. and Britain apparently requires a very special U.S. embassy in London. The new one, opening next month, cost more than $1 billion.

The 12-story glass cube on the banks of the River Thames is the most expensive U.S. embassy in the world. CBS News has been told it's also the most secure — an urban fortress, but specifically designed not to look that way.

Philadelphia based architect James Timberlake says the design was inspired by European castles. It even has its own moat.

"It is not a moat, um, but that was the moniker that was put on it at the outset of the competition," Timberlake said. "It's a pond."

Well, that pond is deep and wide enough to swallow any truck bomb charging toward the building. And that's not the only hidden defense copied from medieval times. Like Windsor Castle, the new embassy sits on a hill, set back from the street by over 100 feet. It will eventually be planted with tall grasses, and a waterfall disguises another defensive wall. Even a planned hedge will hide steel and cement bollards capable of stopping an eight-ton truck.

The 12-story glass cube on the banks of the River Thames is the most expensive U.S. embassy in the world. CBS News

Part of the huge expense was a very modern, 6-inch thick blast-proof wall made of laminated layers of glass that were manufactured in Germany, then sent under guard to the U.S. for further construction, and then shipped back to London.

It is beyond doubt one of the most advanced buildings in the world, but even in comparison to other U.S. embassies, it carries a hefty price tag. Ambassador William Moser says that's the price of security.

"When we build an embassy, it's for a 50 to 100 year commitment," he said. "That's the nature of the business that we're in."

It's the star-spangled neighbor, now just across the river from well-known buildings that really have stood the test of time.