As we discovered in Stage 2 of the walk, Cambridge began on top of Castle Hill, above the muddy river Cam. Slowly, it moved down the hill with Saxon settlements in the river valley around the Quayside and the market square.

Either in the late Saxon period or the early medieval period a ditch was dug going from the Mill Pond at Silver Street, running up Pembroke Street, under the Holiday Inn, down Hobson Street and in a loop back to Quayside. In a way that defined historic Cambridge; the river on one side and the ditch on the other - with a little bit of settlement remaining on Castle Hill.

Beyond the river and beyond the ditch were fields where everybody went to grow their food. Cambridge was hemmed in, and the layout of the fields was established in the late Saxon / early Norman period, so Cambridge is shaped, in a way, by events that happened centuries ago. The town grew up within the ditch, within those confines of the river.

Gradually the ditch got filled in with rubbish and this was the main reason that Hobson's Conduit was built...

In the 1500's there was lots of plague in Cambridge and people were dying. In those days, people were of the belief that if you died, it was because you were a sinner and it was God's way of showing his displeasure. However, disease doesn't care about social standings or class, and just as many of the university staff and students were dying as the townspeople. It was one of the first times when the town and gown were brought together!

They slowly realised that the plague was killing people because of poor sanitary conditions. They realised that the ditch round town was clogged with sewage and rubbish and was a major cause of disease. So in the early 1600's, the town and gown came together to fund and build Hobson's Conduit.

Hobson's Conduit running along Trumpington Street

The conduit brings in water from Nine Wells which are springs at the foot of the Gog Magog hills near Great Shelford. The water comes out of the chalk, and for hundreds of years they filtered into the river at Vicar's Brook. They took over the spring water at the brook and sent it to the bottom of Pembroke Street to try and flush out the ditch to try and keep the water moving. It wasn't terribly successful, but they then had a canal coming in to town and they then used it to take a section of water to the market where they fed the fountain. This provided fresh drinking water for the people which was really important if you lived in middle of town.

Another strand of Hobson's Conduit runs down Lensfield Road, down Regent Street, goes into Emmanuel College and Christ's College to provide water for the pond and you can also see the conduit opposite Christ's College, where people queue for taxis - it's not a broken drain, it's the conduit! It also supplies water for the ponds at the Botanic Gardens and there are some old dipping wells beneath shops like Robert Sayle.

The conduit has an artificial base lined with clay to stop the water seeping away. Some sections are above ground and some below. It's an amazing piece of late medieval engineering and a reminder of how important water is.

The conduit was named after Thomas Hobson. He didn't build the conduit, but he left money to maintain it. His stables were where St Catherine's College is and he was a wealthy entrepreneur who made lots of money on horse traffic between Cambridge and London. The expression 'Hobson's Choice' comes from Thomas Hobson. It means that you've got 'no choice'. Hobson rented out horses for students to go exercising on, and if you went to Thomas Hobson and you wanted to hire a horse, you couldn't have the one you wanted or the best looking one, you had to have the one that had been rested longest by the door... you had 'no choice'!

The reason the conduit head is on Trumpington Road, is because of a fire in the market square in the 1850's. Hobson's Conduit Head used to stand in Market Square. It was a lovely traditional market of sand, mud and scattered stalls. The Victorians used the fire as an excuse to build what they thought was a traditional market. They couldn't cope with a fountain that wasn't in the middle, so they got rid of the traditional fountain - the Conduit Head - and put a Victorian gothic fountain in, which you can still see today.

There is also a deep buried channel that runs under Brown's Restaurant, the Judge Institute, Fitzwilliam Museum and through Downing College. This channel is filled with gravel, sand and mud and is another ancient course of the River Cam. It is probably a continuation of the same channel that runs beneath Jesus College and Jesus Green - the 'grandfather' of the Cam!

Walk along Trumpington Street towards the city centre. You can see Hobson's Conduit running along the side of the road. The Fitzwilliam Museum is on Trumpington Street and is open every day except Monday. Admission is free of charge and you will also find toilets, baby changing facilities and a Courtyard Café.

Keep walking down Trumpington Street, then follow the road round to the left which turns into Silver Street. Walk over Silver Street bridge, where you will see the famous Mathematical Bridge on your right.

As you look at the river, you might be lucky enough to spot a Daubenton's bat. The Daubenton's bat is a medium-sized species that live near water and they fly over the water within a few centimetres of the surface to feed on insects.

You might also be lucky enough to spot a pipistrelle which is one of the UK's smallest bats, or a noctule bat which is one of Britain's laargest!

Keep walking past Queen's College til you reach a grassy area called Queen's Green.