Pazin Kastel (Istria): One of the best-preserved castles in Istria, thanks to renovations, this one is a surprising "must-see" in the Istrian interior. Pazin Kastel is built next to one of the scariest-looking gorges ever, a feature that was conveniently utilized as a dumping ground (literally) for enemies of whoever controlled the castle at the time. What may be Croatia's best ethnographic museum is inside.
Stari Grad (Varazdin): The Gothic Renaissance defensive complex includes a castle and the Varazdin Town Museum. As a whole, Stari Grad is one of this baroque town's best attractions. The museum is an excellent showcase for the town's artwork and historical items. Multi-lingual docents are more than willing to help visitors.
Veliki Tabor (Zagorje Region): North of Zagreb is an imposing solid brick fortress that looks like the place Rapunzel might have let down her hair. Veliki Tabor has its own legends that include murder, mayhem, and a ghost. While the exterior of the 12th-century structure looks like it could withstand a nuclear attack, the inside is still a ruin in search of renovation.
Trakoscan Castle (near Varazdin): North of Zagreb is one of Croatia's most visited sites and one of its most impressive castles -- from the outside. The grounds are extensive and the structure itself is everything you'd expect a storybook castle to be -- stone walls, turrets, a drawbridge -- but inside, renovations have been less than meticulous and sometimes border on the ridiculous. However, Trakoscan is worth the trip if for no other reason than to muse on the plastic deer mounted on the walls outside the entrance.