The massive Fortress on Brimstone Hill, with bastions on multiple levels ascending the hill on which it's built, was started in 1690 and took nearly a century to complete. It was a major pawn in the wars between Britain and France over control of the Caribbean and was occupied by both countries at various times. It was abandoned and allowed to crumble after the Brits and French finally made peace but now it’s meticulously restored to the way it probably appeared in the 1800s.
The must-see Citadel at the summit requires a daunting climb, but the ramp is paved and there are low stone walls you can lean on if necessary. MANY photo ops here.
($5 admission for non-residents of St. Kitts/Nevis. Can be reached only by group tour, rental car or chartered taxi.)