In simplistic terms a broch is a stone tower built in the Iron Age, most brochs were occupied 2000 years ago. The best surviving example is 13 metres tall (Mousa, Shetland Isles), most brochs were probably never this tall.
Brochs are defined by a series of architectural features, entrance passage through thick stone walls which have hollow areas to form stairways galleries and cells, enclosing a central circular area. Some brochs had other structures built outside of them often further houses and rooms. Often there are defensive earthworks (banks and ditches) surrounding the sites.
Most of what we call broch sites survive simply as earthern mounds or are so ruinous that looking for architectural features that define brochs is impossible. There are arguments whether you can call some of these sites brochs when it is unclear if they are or not.
Many broch were later reused, remodelled and /or robbed of their building stone.
Brochs are found in Scotland with particularly large concentration in the North-East.