It's called a "Broch". By Clickimin loch near Lerwick, it was built to keep marauders, who might be the neighbours or Vikings, or both, at bay until they lost interest.
Presumably they worked, since many were made all over Scotland and the islands. Originally much taller, such structures have provided the resources for later ones if the habitation is continuous. That's why Archaeologists like really big cataclysms. The Broch, like most of the Shetland excavations, have been tidied up but not rebuilt, the stones are where the Picts put them.
This is an iron-age archaeological site uncovered by the construction of the new road for the airport. Lots to look at and listen to. Really good educational stuff to give a glimpse of what life was like. The Team there has built a replica house seen here in front of the (early 21st cent) car park.
The weather was a tad unsettled that day and the advantages of a proper stone building became more obvious as the legs got colder and the meaning of "wind-chill" clearer. The Archaeologists seemed to take it in their stride, but you can see why they don't mind hunkering down behind a wall and digging out a small pit rather than standing on top and taking the broad view. The visitor center has heating, which was greatly appreciated by the less sturdy tourist, and lots to look at, with an excellent sections on weaving and stone working. Also a chess like game called Hnefatafl which Terry Pratchett must have heard of when he invented Thud. Here's a link to a page about it that I found using Google, I'm sure there are many others.
An advantage of staying at the Sumburgh hotel (in the background) is easy access to Jarlshoff. Of neolithic origin perhaps 7000 years ago when the islands were thought to be first inhabited, and occupied into the sixteenth century. Presumably built for easy access to the airport.
Incidentally, but quite unconnected to the archaeology, much of the aircrew and allied trades stay at the Sumburgh Hotel when stopping overnight. Don't irritate them for obvious reasons, but if they look unusually nervous and keep biting their hat-brims, it might be worth taking a closer look at the weather report. It would be especially advantageous to delay your flight by a day or two if you hear them using "Banter". Never a good sign in pilots, it was noticable during the second world war and based on the good old British understatement, and demonstrated by expressions like "a bit tricky" when landing with no undercarriage
Some Archaeology we saw in Shetland