New US embassy in London has secret security features

The new US embassy in London is the most expensive in the world - and the most secure.

Located on the banks of the river Thames, the 12-storey building cost US$1 billion (NZ$1,428,571,428). It's an urban fortress, but has been specifically designed not to look that way.

Architect James Timberlake took inspiration from European castles when designing the embassy. It even has a moat, although Mr Timberlake prefers the term 'pond'.

The moat is more than just a design feature - it's deep and wide enough to foil truck bombs heading for the building. It's just one of the embassy's many hidden defences.

Steel and cement bollards capable of stopping an eight-tonne truck will be hidden by a hedge, while a waterfall disguises a security wall. 

The embassy is covered by a six-inch thick 'blast-proof' wall of laminated layers of glass, the material for which was constructed in Germany and further improved in the US, before it arrived in London.

The building's extra security measures weren't cheap, but ambassador William Moser says the price was worth it.

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