Definitely my favourite Castlevania on GBA.
You start as a Nathan Graves who with two Baldwin family members enters Dracula's Castle. In effect of accident is separated from others and need to fight his way out of castle and save his comrades. Game have an old-school touch of "old-pre-SotN-Castlevanias". Your basic weapon is good old whip (that you will stick to for rest of the game) so forget about changing weapons or shields like in "SotN". But it's not so bad. You have some additional weapons (that usage depends on number of hearts) and DSS (Dual Step-Up System) that can give you power ups from combination of special cards that you get form enemies.
Few words about move list. First of all one of effects of this "old-school touch" is that you can't run from the beginning and you need to get special ability (after killing first boss) and then you need to press "forward" double time to run. It can gets on the nerves sometimes. From good things - list of basic abilities is divided and don't depends on cards or something (that is really annoying in "Aria of Sorrow"). From bad things: forget about changing into bat.
What I liked in this game is that it's not an easy task. First half is quite easy for an experienced player. However after it you can get sweat a bit in some locations. Also Final Boss is a hard one and to survive you need to show skills and not only leveled up character. Big advantage is style and design. It's quite close to "SotN" and have a bit of old style horror atmosphere that is very pleasant. Graphics are very well done for a one of first games on GBA. Music makes nice atmosphere (menu music is great) bringing on mind old-school melodies from SNES. But it's not irritating and just fit as a background.
Game isn't long. If you want only to get through it it will take you less than 10 hours but there are an additional secrets and tasks like Battle Arena which can take you some more time. Game isn't complicated and puzzles are quite easy but still basic fun is from killing Dracula's servants.
So if you look for good "Castlevania" game on GBA, I think this one will be the best choice. Give a challenge and a lot of satisfaction combined with nice design and great atmosphere. Worth trying.