The mention of British military establishments in the context of colonialism can stimulate a wide spectrum of reaction. However, for those willing to leave the conventional tourist circuit and, with open mind, visit Malawi's once-proud fortresses, the rewards can be substantial. History becomes almost tangible. Some of the remoter sites are visited so infrequently that spent cartridge cases may still lie where they fell or were dropped over a century ago. Many offer breathtaking views over miles of serried valleys or distant waters reflecting Morse-code flashes of sunlight.

Given its size, Malawi is rich in forts - 60 sites have been identified. The country has few natural resources - certainly none profitably exploitable by a foreign power. So why the former British presence? The answer, as with much in Central Africa, can be traced back to David Livingstone and the slave trade. In the mid-19th century, when Livingstone confirmed the existence of Lake Malawi to the outside world, he also brought attention to the thousands of Africans being abducted to feed the insatiable Arab slave markets of the Swahili coast. In response to his call for action, the Universities Mission to Central Africa was later formed to spread Christianity and promote commercial alternatives to slave raiding. Soon an unwilling British Government found itself with a Protectorate to administer and engaged a small contingent of British officered Sikhs from the Indian Army as an embryonic police force, under Captain Montgomery Maguire.

Apart from imposing the Pax Britannica and halting traditional inter-tribal raiding, the key military objective was to dismantle forcibly the centuries-old slave routes. The three forts chosen here among the many built to enforce this initiative amply illustrate the resolve and determination of a few men and their local allies, who would soon become the King's African Rifles. All three forts are accessible as day trips from Malawi's main tourist centres and are well worth the varying degrees of effort required to reach them.

Situated on rising ground 600m from the eastern shore of Lake Malawi, Fort Maguire is in an area little visited by tourists, near the Mozambique border. Even in its ruined, overgrown state, it's evident that the fort had a clear view across the lake's narrowest crossing point. From the cove below, Arab dhows ferried their human cargoes across the water before the long, forced march to the coast.

In 1891, Captain Maguire led a small assault force of 28 Indian soldiers into the cove to destroy two slaving dhows belonging to the local chief, Makanjila, but Makanjila arrived with 2000 followers. Fighting a desperate rearguard action, Maguire sustained a fatal gunshot wound to the head as he swam in vain for his life. It was another two years before the chief's villages were neutralised and the fort built. Makanjila attacked it in 1894, but was repulsed, suffering heavy losses. With vestigial perimeter walls of local stone measuring 300x200m, the remains of the brick-built guard house and the officers' accommodation can still be made out among the encroaching bush. The old parade ground of packed, parched mud remains largely devoid of vegetation, save some stately baobabs and a scrubby depression. Amateur diviners approaching this depression with a forked stick can confirm the presence of what's probably a long-collapsed well. Given a solitude interrupted only by the sounds of the bush and birdlife, it's difficult to imagine the crash of gunfire and screams of warfare heard here just over 100 years ago. Enamoured with the site's history, I once camped under the stars on the sandy beach where Maguire met his end, only to be woken by passing villagers as dawn broke. Shaking their heads, they showed me, just metres away, unmistakable marks in the sand where a crocodile had left the lake during the night.

In contrast to Fort Maguire and the humid, lakeside atmosphere of the Rift Valley, Fort Lister enjoys a more bracing environment on the northern slopes of Mount Mulanje (at 3002m, Central Africa's highest peak). Built in 1893, the fort was named after Sir Villiers Lister, a strong opponent of slavery. The Fort Lister Gap, a steep mountain pass between Mount Mulanje and neighbouring Mount Mchese in the extreme south-east of the country, was a favoured slave caravan route into Mozambique. Built on a low plateau with a commanding view of the pass and its approaches across the Phalombe plain, Fort Lister also served as the base for the local District Officer and even had a Post Office.

The approach to the fort is signposted from the steep, stony track rising from Phalombe, and involves a leisurely walk along undulating forest tracks and across a stream. The fort's outer defences are still clearly defined, as are the ruins of the guardroom, stores and prison. Scouting around in the surrounding forest will uncover further, creeper-clad ruins, including those of the District Commissioner's bungalow. Within the walls are two lonely graves, one belonging to Gilbert Stevenson, cousin of the author Robert Louis Stevenson, who shot himself in unclear circumstances in 1896.

The most spectacular of all Malawi's forts is undoubtedly Fort Mangochi, due not least to its elevated position, in a saddle between two peaks, commanding panoramic views of Lake Malawi and distant mountains. The site was once the stronghold of Chief Zarafi, a notorious slaver and enemy of the British, whose township contained around 25,000 people in the 1890s. Today this remote mountain fort, built on the site of the defeated Zarafi's compound, is in a Forest Reserve; reaching it requires vigorous exertion. In February 1896, a Lieutenant Alston noted in his diary on arrival: "It has been a fearful long day's march and at the end of 26 miles one has to climb right to the top of Mt. Mangochi… almost perpendicular! Fearful… There is another white man here, an ex-corporal Royal Engineers, who drinks gin and whiskey all day and has eight native wives!"

The most direct route up requires a steep 650m climb, but the rewards are gratifying. Although in ruins, the fort is protected by wire fencing to prevent further demolition by inquisitive elephants. The administrative buildings, commanding officer's quarters and barracks are readily identifiable. Substantial perimeter walls, once stockaded, emphasise the post's importance, although it was clearly vulnerable to gunfire from surrounding hillsides. As with much game viewing, these forts are best approached at the end of the dry season, when grass and vegetation have died back (exploration can be almost impossible during the rains). Venture in search of the men who freed Malawi from slavery and you'll be rewarded with an experience that may elude future generations, as nature inexorably reclaims lost territory.