The river Thames in the Kingdom of England ( ~950 AD -> now +/- various other countries ) usually was freely flowing - just as the article states.

The article takes a long view of time, stretching back to at least the founding of London, the capital city, two thousand and more years past on the banks of that very river.

There was a relatively short period of time when the Thames did freeze in winter and England was much colder, this article talks about a chunky bit of architecture during that period.

The windows are discussed wrt their thermal effects, allowing the sun in to heat central stone during the day on one side of the building, likely with heavy curtains at night, with windows blocked internally and largely for show on the rear.