London Bridge doesn't have a 'top'. If you are referring to the walkway at the top of Tower Bridge, it was intended to allow pedestrians to cross the bridge even when it had been opened to allow river traffic to pass underneath it.
The London bridge
London
The London bridge was built in Arizona in the year 1831. This is built over Lake Havasu.
London Bridge was built (to replace a bridge having been built by the Romans) in 1176. It was being built for 33 years. Until 1750, it was the only bridge which crossed the Thames. The modern London Bridge was built in 1973. (The original one was sold to an American businessman.)
Tower Bridge
The current London Bridge is a pre-stressed concrete box girder bridge.
The first London Bridge was built around 50AD by the Romans.
The original bridge was built by the Romans
it was built in London, England over the Thames river
The 1973 London Bridge is a pre-stressed concrete structure.
The first London Bridge was built by the Romans around 50 AD
There have been many London bridges. The modern London Bridge built in 1971. It was designed by William Graham Holford, Baron Holford and built by the firm of Mott, Hay and Anderson. John Rennie designed the London Bridge of 1831-1967.