Uninspired game with lots of backtracking.
What you do is running through one huge maze with monsters in every room. Since you don't know the way in advance you go a lot forth and back to the save room and forth again to find a better route. Since the monsters respawn every time you enter the room you have to slay them over and over again. There is a hand full of puzzles to mix up the gameplay a bit but they are just not enough, they are too obvious and the rewards are meager. Occasionally you enter a room with another tiny bit of the story but it isn't really engaging either.
What makes the game challenging are the boss fights. They often seem unfair and require a lot of practice but most times you get a cool new skill once you've beaten a boss.
The good part of Castlevania: DoS is that you can equip monster souls which you can get from defeating monsters and you can choose from good variety of weapons. The game lets you customize your fighting style pretty well this way.
Graphics and music are, like most times, a matter of taste. The style is a little antiquated and reminds of SNES / Genesis zombie slaying games. Most of your time you spend inside a castle. So expect a lot of dark stone walls and candleholders throughout the whole game. The music sounds like organ.
If you like hard boss fights, zombies and other classic horror creatures check out the game. If you don't like this stuff get something else since the gameplay alone isn't worth it.