Karlovac sits at the confluence of four rivers — the Kupa, Korana, Mrežnica and Dobra — and was built in 1579 as a Renaissance star fortress to defend the Habsburg frontier against the Ottomans. The original six-pointed star grid is still visible from the air, and walking the old town's streets you can feel the military precision of its layout.
Today Karlovac is Croatia's fourth-largest city and a pleasant place to stop between Zagreb and the Dalmatian coast. It is not yet on the tourist trail in any serious way, which is part of its appeal. The old fortress town (Stari Grad) has a relaxed, lived-in feel — people actually live and work here rather than putting on a show for visitors. The medieval Dubovac Castle on the hill above gives a panorama of the whole.
The four rivers are a genuine asset. Swimming beaches along the Korana — concentrated at Foginovo kupalište — are popular with locals in summer, and kayaking on the Mrežnica is excellent. The riverbanks are lined with old mills and the water is clean. If you have a bicycle, the riverside paths connect to some beautiful countryside.
Karlovac is also the home of Karlovačko beer, one of Croatia's best-loved lagers, brewed here since 1854. The brewery does not currently offer public tours, but the beer is everywhere and worth drinking where it's made.