Retro Studios deserves an ice cream party. There newest foray into the Metroid universe, Metroid Prime 2: Echoes, may not seem to different from Metroid Prime, but what it lacks in new isn't necessarily a bad thing. MP2:E plays every bit as good as the first, but with a great new story line, great new baddies, great new levels, and some unique trans-universe puzzles. The splitting of the planet makes for interesting game play that hearkens back to some Zelda or Banjo-Kazooie games (of which I can't quite put my finger on), but in a 3D spooky way. If you liked Metroid Prime, you must get a copy of Metroid Prime 2: Echos.
Update: Castlevania: Harmony of Dissonance had the 2 castles with some interaction. However MP2:E has much more interaction than C:HoD did, which I always felt was underused in that title.
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Disclaimer The opinions expressed herein are my own personal opinions.