Different from the rest of the series
This one was set apart in that you weren't spending time on a 2D plane just going from one level to the next. In this one you actually were "traveling". There are towns to visit, people to talk to, weapons to upgrade, and secrets to discover. This in a sense gave the game an RPG feel.
Another revolutionary bit for this series was the aspect of "time", that being the game went through night and day cycles. This could effect things, mostly what monsters were on the field. The nighttime creatures were obviously more powerful and harder to kill. Thankfully this was rememdied over time with the upgrade of weapons/equipment.
Storyline wise, you are given a threadbare plot to keep you trekking along to the finale.
Overall, one will find this a nice distraction in what I consider the best game of the Castlevania trilogy on the NES.