The Bar/Lounge at Hotel Central
Hotel Central's bar/lounge is recommended by reviewers writing for major publications. Out of 5 professionals tracked on TripExpert, 2 praised the bar/lounge, like Frommer's and DK Eyewitness.
Hotel Central 
Frommer's
DK Eyewitness
Frommer's "Room prices are moderate rather than bargain-priced (for Zagreb), but you're paying for location"
DK Eyewitness "There is also a bar, though it is rather uninspiring"
