Even on a summer's day, when fluffy clouds scud across a blue sky and the high moor looks particularly lovely, Princetown is bleak. There's nothing soft and gentle about the place – most of the buildings are functional in the extreme, uncompromising, grey and harsh. The town, 1,395ft (425m) above sea level, and with an average annual rainfall of 82in (2,160mm), was founded by Sir Thomas Tyrwhitt in the late 18th century, and named in honour of the Prince Regent, to whom he was both a friend and private secretary.

Dartmoor Prison

Tyrwhitt persuaded the government to build a prison here for French prisoners from the Napoleonic wars. Building work started in 1806, and the first prisoners were in situ by 1809, joined by Americans in 1813. At one time 7,000 men were held. Closed in 1813, the prison reopened in 1850 as a civilian establishment, which it remains to this day – a monumental building, best seen from the Two Bridges to Tavistock road, to the north of the town.

There is mention of the ancient landmark of Nun's Cross (or Siward's Cross) as early as 1280, in documents concerning ownership of Buckland Abbey lands. Over 7ft (2.1m) high, it stands on the route of the Abbot's Way – between Buckfast Abbey and Tavistock – and marks the eastern boundary of Buckland Abbey lands. The word 'Siward' engraved on its eastern face may refer to the Earl of Northumberland who owned much land in this part of the country in Saxon times, or may indicate some connection to a family named Siward who lived nearby. 'Bocland' on the other face may be a reference to Buckland Abbey. The word 'Nun's' comes from the Celtic nans, meaning combe or valley.

The Devonport Leat is an amazing feat of engineering, carried out between 1793 and 1801 to improve water supplies to Devonport, now part of Plymouth, which at that time was being developed as a naval base. Originally 26.5 miles (43km) long, it carried 2 million gallons (9 million litres) of water a day. Lined with granite slabs and conveying crystal-clear, fast flowing water, today it provides an extremely attractive, level walking route through some otherwise fairly inhospitable terrain. The final part of the walk, back to Princetown, follows the abandoned railway track that Tyrwhitt planned to link Princetown with Plymouth. The line, the first iron railway in the county, opened in 1823. More of a tramway than a railway, the horse-drawn wagons carried coal and lime up from Plymouth, and took stone back. In 1881 commercial considerations caused the line to be taken over by the Princetown Railway Company. It reopened as a steam railway in 1883, until its eventual closure in 1956.

Leave the car park past the toilets and turn right to pass the High Moorland Visitor Centre. Cross the road and follow the lane to the left of the Plume of Feathers Inn. After 100yds (91m) a small gate leads to a broad track which ascends gently to South Hessary Tor, from which there are splendid views to Plymouth Sound, ahead, and of the prison behind.

Follow the track as it drops gently, passing boundary stones. It crosses two other tracks (after the first look right to spot the Devonport Leat) before descending to Nun's Cross (2.5 miles/4km from the start). Nun's Cross Farm (originally thatched c.1870) can be seen to the left.

Turn 90 degrees right at the cross to pick through old tin workings to find the tunnel where the leat emerges. It's near the ruins of a building under a beech tree. Walk along the right bank of the leat, soon passing a granite cross.

Just before a bend meet a track and cross the leat via Older Bridge. Continue along the left bank, with wonderful views of Burrator reservoir to the left. Cross back to the right bank before descending to the valley of the Meavy. A good option is a bridge by blocks of worked granite as the Burrator reservoir plantations come into view. The descent to the Meavy is steep and rocky.

Cross the aqueduct; the leat turns left. Take the grassy path right leading slightly uphill away from the river (there is a wealth of tin working evidence in the valley – worth an exploration). Bear left and climb steeply up to Black Tor on the hilltop.

Go straight on past the Logan Stone, one of several on Dartmoor balanced in such a way that they can be rocked on their base, and on across open moorland to the road, with views of Brentor, Swelltor Quarries and the disused railway line ahead. Turn right at the road.

Pass a blocked-off parking place and continue along the verge. The road descends past a small parking space; as it rises again bear left by a granite block across rough grassland, aiming to the right of the mast on North Hessary Tor.

At the railway track turn right towards the edge of town. Keep left at a fork, eventually passing a small gate to join a road, with Dartmoor Brewery left. Bear right to the car park.