So, the way you've written this comes across as if you think that's a canal running across midsummer common. For the benefit of those unfamiliar, it's not a canal, it's the River Cam. It may well be that the reason for no fences is boat transport, though.

There definitely was serious boat transport in the area- midsummer common hosted a fair; historically fairs were actually vital to the economy as a lot of goods were exchanged there, and even more contracts for goods. Nearby Stourbridge fair was the largest in Europe: https://en.m.wikipedia.org/wiki/Stourbridge_fair and the inspiration for Bunyan's "Vanity fair" in "Pilgrim's Progress"