Brochs generally dated to late 1st mill bce/early 1st mill ce.
There are several still in good condition - mainly in the islands.
Mousa on Shetland is the best.
http://www.themodernantiquarian.com/site/3989
There are several in Orkney including Midhowe
http://www.themodernantiquarian.com/site/598
Another good one at Dun Carloway in the Hebrides
http://www.themodernantiquarian.com/site/791
On the mainland, there are two good ones at Dun Telve and Dun Troddan in the west
http://www.themodernantiquarian.com/site/792
http://www.themodernantiquarian.com/site/793
and a not bad one at Carn Liath in the east
http://www.themodernantiquarian.com/site/5118
From these photos, you can get a good idea of the construction.
The walls are partly solid, partly double-skinned allowing for intra-mural passages and stairs. Entrance is through a defended ground floor door.
The Orkney sites show the best surviving use of the internal space (because stone was used) but some of this was added after the main period of broch use.