Diên Khánh Citadel is an 18th-century Nguyễn fortification inland from Nha Trang, built with earthen ramparts and brick gates rather than the stone drama visitors might expect from a European castle. Several gates survive, and the outline of the old defensive walls is still readable in the town.
It is a good half-day detour if you want a break from beaches and boat trips. The citadel is not heavily interpreted, so it helps to arrive with a little background on the Tây Sơn wars and the Nguyễn lords. Pair it with a wander through Diên Khánh town and treat the visit as a glimpse of provincial Vietnam rather than a polished museum stop.