English Heritage invites you to take a winter walk around its sites. Mervyn Hancock visits Old Sarum in Wiltshire.

I f you haven't managed to get rid of some of that extra weight accumulated after the Christmas and New Year, then a trip to Old Sarum could be the perfect way to revive your senses.

Swap the television and those brain-training games for a pair of boots and a warm jacket, and head off to one of Wiltshire's most visited sites.

Perched high above the Salisbury Plain, Old Sarum is steeped in history. Originally an Iron Age hill fort, it was occupied by the Romans, Saxons and Normans before becoming a thriving medieval settlement.

You can literally travel back in time while enjoying a refreshing walk on one of the many footpaths, or even ask English Heritage staff on site for recommended routes. You can also reach the castle ruins by bike via the National Cycle Network paths.

At first glance, Sarum may seem an unlikely location for a settlement, let alone a royal castle. A 13th-century poem by Henry d'Avranches describes it as "a windy, rainswept place without flowers or birds... the bare chalk dazzled the eyes... There was a shortage of water, and a tiring climb to the top of the hill."

The site is approached via an opening in two high Iron Age banks, which obscure the ruins from outside and give it the air of a mysterious hidden castle. The banks were begun almost 5,000 years ago, and remained intact until the Roman invasion. The Romans installed a garrison in the river valley below the site and it was probably used as a market centre. At this time it was called Sorviodunum.

The fortress had one supreme advantage. In time of war, and rumour of war, its quality as a defence against invaders was unrivalled in the neighbourhood. It commanded the whole area; no one could approach without being observed; it was protected on three sides by steep banks; and it dominated the River Avon and the Roman roads.

A royal palace was erected within the banks in 1130, and the first cathedral on the site was completed some 40 years earlier, but it burned down only five days after it was consecrated. A new, larger cathedral was completed in about 1190. But relations between the clerics of the cathedral and the castle guard were punctuated with outbreaks of petulance and occasional violence.

The churchmen became so exasperated that in 1219, Bishop Richard Poore decided that enough was enough and he determined to build a new cathedral at a location several miles to the south. A settlement grew up around the site of the new cathedral, and it is this settlement that is the modern city of Salisbury.

With the shift of settlement away from Old Sarum to New Sarum (Salisbury), the old site lapsed and the castle fell into disuse. Despite the fact the site was derelict, Old Sarum continued to send a representative to Parliament until the mid-19th century.

Today, visitors can see remains of both the castle and the second cathedral, though little of that building exists beyond an outline of the walls on the grass. English Heritage, which administers the site, will arrange special guided tours for groups who book ahead.

Take time to admire the immense labour and effort that was needed to construct the banks and ditches that surround the hilltop. Built with only crude spades and buckets by Iron Age people, Old Sarum is an impressive feat of engineering before the age of mechanised cranes and diggers.

Naomi Matthews, visitor operations manager, said: "We're all a bit guilty of overindulging during the holidays, which is why New Year is a great time to turn over a new leaf and make the most of the stunning Wiltshire countryside on our doorstep.

"Old Sarum has been part of the landscape around Salisbury for thousands of years, it offers up a wealth of fascinating history and gives people the opportunity to brush off the cobwebs and enjoy a hearty walk around the massive Iron Age ramparts."

Old Sarum is situated two miles north of Salisbury, off the A345. Site opening times: until February 28, 2009, 11am-3pm daily; March 1-31, 10am-4pm daily.

Admission is free to English Heritage members. Non-member prices (until March 31, 2009) are: adults £2.90, children £1.50, concessions £2.30.

For more information about Old Sarum (including accessibility), call 01722 335398 or visit www.english-heritage.org.uk/oldsarum