There was no glass for windows in those days, or, at least, very little.

It had been introduced, it is said, in one instance, and that was in a monastery in the north of England.

The abbot, whose name was Benedict, brought over some workmen from the Continent, where the art of making glass windows had been invented, and caused them to glaze some windows in his monastery.

It was many years after this before glass came into general use even in churches, and palaces, and other costly buildings of that kind.

In the mean time, windows were mere openings in stone walls, which could be closed only by shutters; and inasmuch as when closed they excluded the light as well as the air, they could ordinarily be shut only on one side of the apartment at a time--the side most exposed to the winds and storms.
Alfred accordingly found that the flame of his candles was blown by the wind, which made the wax burn irregularly; and, to remedy the evil, he contrived the plan of protecting them by thin plates of horn.
Horn, when softened by hot water, can easily be cut and fashioned into any shape, and, when very thin, is almost transparent.