The window tax was a glass tax which was an important social, cultural, and architectural force in the United Kingdom during the 17th and 18th centuries.
Glass making was costly and the use of glass for windows and other purposes was even costlier because of a tax levied specifically on it. The tax was introduced in 1696 under King William III of England and was designed to impose tax relative to the prosperity of the taxpayer. The bigger the house, the more windows it was likely to have, hence the more tax the occupants would pay. This is the reason for the bricked-up windows so often seen in very old houses in the UK.
The richest families in the kingdom used this tax to set themselves apart from the merely rich. They would commission a country home or a manor house whose architecture would make the maximum possible use of windows. In extreme cases they would have windows built over structural walls. It was an exercise in ostentation, spurred by the window tax.
The tax was not repealed until 1851, when it was replaced by a tax akin to the present-day "council tax".