Kostolec is an archaeological hilltop fortification near Ducové, northeast of Piešťany. It is a modest site rather than a reconstructed attraction, but it gives the spa region a much older layer than the villas and bathhouses in town.
Go if you have a car, a bicycle, or a strong interest in early medieval Slovakia. The reward is as much the position above the Váh landscape as the remains themselves, and it pairs naturally with a broader drive through the villages around Piešťany.