Don't be drawn to the town's spectacular medieval castle, flaunting itself from the high ground to the east of town. What you are looking for was not meant to be visible from sea-level. Hidden, not 400 yards from where you are standing, up on the hills to the west, is a sprawling complex of 19th-century fortifications sunk into the grassy slopes.
Originally put up in the 1780s to defend Britain against invasion from the Continent, the Western Heights defences were progressively reinforced during the following century as Anglo-French tension continued to ebb and flow across the Channel. When finished, they formed a fortified line a mile-and-a-half in length. Most of it has long since been abandoned, leaving plenty of deep overgrown ditches, dilapidated drawbridges, tunnels and chambers, and chalk footpaths to explore.
Before you begin your assault on the Western Heights - as they are known - call into Dover Museum in the Market Square. From here, simply head up Durham Hill opposite the White Cliffs Experience along York Street. You won't miss it: just follow the signs for the Western Heights Trail, Drop Redoubt and the Young Offenders Institution. After a few minutes, climbing quickly, Dover soon retreats to a sea of rooftops and the muffled shouts of children.
There is no set way to tackle these defences. On an information board at the top you will find a sign with a map of trails inviting you to explore. I set off around the perimeter following a thin line of chalk, worn through the grass by previous visitors. But it wasn't long before I was tempted to deviate off the waymarked paths, lured by the words "way in" chalked on to a rock or fencepost by the local kids who have made this their adventure playground.
In places, the Western Heights are like a lost world. Sections of the ditches are so overgrown that they encase you in a canopy of thicket. All you can see is the narrow path threading through the undergrowth, allowing glimpses of the ivy-covered brick and flint walls punctuated with narrow concrete window slits. It is shady and very quiet, except for when you disturb small animals and birds. There are dead-ends, hidden flights of steps, bolted doorways and even poisonous serpents. Then you suddenly arrive at a clearing and there is a hole chiselled into the wall. A way in.
It is at this point that you begin to see why this is an adventure. By definition, adventure is something that contains an element of danger. And to quote the disclaimer from the guide book by David Burridge: "... it is possible to get in here BUT this does not mean that you have the right to do so; it is not necessarily safe to do so; and don't blame me if anything goes wrong".
When I found myself at one such opening near the detached bastion, curiosity overcame reason and I squeezed feet-first through the child-size opening, dropping into the darkness. I stood still until my eyes had adjusted and the unnerving feeling of being somewhere I shouldn't be had subsided. Looking around this first room I could see the dangers were real enough. The floor was strewn with broken masonry and I could see the vaulted brick ceiling through gaping holes in the first floor.
Moving tentatively from room to room negotiating crumbling stairs and narrow, sloping passageways, it would be easy to fall foul of one of the counter-balanced drawbridges that form a second line of defence on the inside. But after disturbing a few birds it didn't take long to get spooked and feel the need for daylight.
Back outside, the perimeter trail leads you past the Young Offenders Institution until you reach the western fringe of the Heights, where the route becomes a little unclear. One path heads downhill towards the sea and then back along the southern side of the fort, but you don't see much of the ruins if you head this way.
The other choice follows the edge of the ditches, picking through a thatch of unforgiving hawthorn which is made all the worse by the adverse camber of the narrow trail. The reward for choosing the latter comes when you emerge on the lip of a 60ft drop overlooking the face of the Western Outwork - a row of elegant red and yellow brick arches wedged into the chalk cliffs.
Whichever route you take should lead to the gun emplacements at St Martin's Battery and to Drop Redoubt, the detached fortress back at the eastern end of the complex. These are the most visited parts of the fort and in recent years the local authorities have provided information boards and even started some restoration work, bricking up holes and clearing ditches.
Of course, restoration is no bad thing, but untangling the intrigue of the ditches and sealing any chance of entry does lessen scope for adventure. Then again, if they fall any further into neglect, the Western Heights will face their toughest battle yet: against nature.
By train: departures twice hourly from London Victoria to Dover Priory; adult cheap day return pounds 17.80.
By car: the most convenient town-centre car-park is on York Road; the closest to the defences is on North Military Road.
Guide to the Western Heights by David Burridge is available from the Dover Museum shop (pounds 2.95). A collection of material relating to the Heights and guided group tours of Drop Redoubt are available by appointment. The museum opens daily during the summer, 10am-6pm, adults pounds 1.60, children 80p (tel: 01304 201066).
Dover Castle has a network of wartime tunnels burrowed into the chalk cliffs. Unlike at the Western Heights, commentary, film and even smells are provided so that you can relive the Second World War in the anti- aircraft operations room, military surgery and communications station. A guided tour of the tunnels is included in the entrance fee: adults pounds 6.60, children pounds 3.30, family ticket pounds 16.50. Dover Castle is open daily in the summer, 10am-6pm (tel: 01304 201628).