An iron age broch from around 2000 years ago.
Unfortunately some of the stones from this broch were taken to build the Bernera Barracks: www.ipernity.com/doc/davidyork/51291936

Unusually close to another broch: www.ipernity.com/doc/davidyork/51295966

More photos of this broch:
www.ipernity.com/doc/davidyork/51295938
www.ipernity.com/doc/davidyork/51295940
www.ipernity.com/doc/davidyork/51295942