Actually, that's a common misconception. The posts appear to be to the side, but they actually rest on the boats. Here: http://scribalterror.blogs.com/scribal_terror/images/2008/05/19/p98y.jpg
No. The bridge stood on a number of artificial islands supporting the base of each arch. They appear boat shaped because that is the natural shape to reduce turbulence both upstream and downstream. These isalnds (I think they werre called 'starlings') took up so much of the width of the river that the constrained the tidal flow of the Thames. Passing under the brdige through one of the gaps in the starlings was a dangerous business because of the strength of the river flow.
also, you state that the clock tower, Big Ben was built in 1288. This is incorrect it was actually built6 in the 19th century, as was the currrent houses of parliament. The only bit that would have been around in the middle ages is Westminster Hall.
Important buildings from around that time would be, St Paul's Abbey (not the current one), Westminster Abbey, Westminster Hall as mentioned previously, Tower of London, and of course London Bridge.
You will also want to include markets, lots os shipping (London was a major port), churches and other religious institutions such as priories and monasteries.
Good luck