A tower in, in effect, the medieval skyscraper writ smaller (due to the limitations of technique).
Circular or square, towers were generally stone-built. On their own they form Norman-period keeps or middle-period Anglo-Scottish pele towers. A similar construction earlier in history in Scotland, was the broch, which had double walls, with a stair between them, which offered a primitive system of insulation.
The virtue of the tower was that it offered multiple use of the same square footage of ground, and piling more people and things into the same area which only had to be defenced once. It also offered a height advantage to those defenders over any attackers. Towers became commonly used in the curtain walls of castles, as strongpoints, and to protect corners and apertures like gateways.
The "Tower" in London
The Tower is a term used to describe the White Tower of London. Used initially as defence and later as a prison. Located originally on the banks of the Thames River it now sits a distance from the river due to the filling in off the banks.