Luxembourg has around a hundred castles and castle ruins across 2,586 square kilometres — one of the highest densities in Europe. The reason is its position: for a thousand years this was contested ground between France, the German states and the Low Countries, and every valley crossing was fortified.
Bourscheid
Bourscheid is the largest and the most impressive: a hilltop enclosure of eleven towers and a curtain wall on a spur 150 metres above the Sûre, with views in three directions. Founded around the year 1000, abandoned in the eighteenth century and consolidated as a ruin from the 1970s, it is presented honestly as a ruin rather than over-restored.
Beaufort and Larochette
Beaufort pairs a medieval ruin with an intact Renaissance château beside it, both visitable, in the Mullerthal region.
Larochette sits on a sandstone crag above its village, with two partly restored manor houses inside the walls, and is one of the more atmospheric ruins because the village below has changed so little.
The Valley of the Seven Castles
The Vallée des Sept Châteaux follows the Eisch river west of the capital and takes in Koerich, Septfontaines, Ansembourg — which has two, a ruin above and a seventeenth-century château with formal gardens below — Hollenfels, Schoenfels and Mersch.
A marked walking and cycling route runs the length of it, thirty-seven kilometres, and it makes an excellent day out of the city.
Others
Vianden is the grandest and has its own article's worth of history. Clervaux holds the Family of Man exhibition. Useldange has been fitted with walkways designed for blind and partially sighted visitors, with tactile models and audio — an unusually thoughtful piece of accessibility work.
Practical
Most castles charge a small admission and open from spring to autumn, with reduced winter hours; several close entirely in January. A combined ticket covers some of them.
Public transport is free and reaches the main sites, but the smaller ruins are much easier with a car. Distances are tiny — no castle in the country is more than an hour from any other.
Why so many
The density comes from fragmentation: for centuries this was a patchwork of small lordships, each requiring its own fortified seat, sitting on the fault line between French and German spheres. Most were built between the eleventh and fourteenth centuries, damaged in the wars of the seventeenth, and finished off by the dismantling of the Luxembourg fortress after 1867.