Like Relic Passage, Castelia Park is a relatively tiny offshoot of the Castelia Sewers. Indeed, it's much smaller than Relic Passage, and seemingly less notable as the setting for its own guide. Relic Passage is, ultimately, much bigger.
For the moment, though? Castelia Park is much better. Sure, you'll get a few pokémon in Relic Passage that don't appear here, but they're not nearly as interesting - or useful to your current goals in the game.
Reaching Castelia Park
Castelia Park is quite easy to reach from Castelia Sewers, though it's also easy to miss. When you first enter the Sewers you'll see the niche where you encountered the members of Team Plasma to your northwest; just south of that is a pathway that branches south and east. To the east are stairs. On your first trip through the Sewers, with Hugh, a man will be standing in front of the stairs, blocking your path through.
Solution? Ditch Hugh. Beat up the members of Team Plasma. Once Hugh's gone, you can go up the stairs and find Castelia Park.
Why's it important?
There's not much to do in Castelia Park other than range through the grass. Grab the Miracle Seed, talk to the man, and you're done. Right?
Wrong. The grass, light and dark, is the primary reason to come to Castelia Park. There are a lot of really cool pokémon here, some of which you won't find anywhere else in Unova. Here's a brief rundown of what you can find.
- Audino (rustling grass). Yes, you can find them anywhere, but at this point in the game this source of Audino is the strongest. If you're going to train for Burgh's gym, this is where you should do it.
- Eevee. These little normal-types don't seem all that special, but they have the distinction of evolving into seven different types of pokémon, each with a distinct type and powerful in its own right. You could make an entire team out of Eevee-volutions. Eevee are rare, so expect to look around a lot.
- Pidove. Nothing special, but always useful if you need a flying-type.
- Rattata - Nothing special. You can find them in the Sewers.
- Buneary. Another normal-type that turns into a half-decent fighter, and a good jac-of-all-trades for covering types you don't otherwise have.
- Cottonee. A grass-type. Not that amazing, but defensively solid and a good choice if you didn't take Snivy as your starter.
- Lopunny (rustling grass). The evolution of Buneary. Why bother with the preevolution, right?
- Whimsicott (rustling grass). The evolution of Cottonee. Cut to the chase!
- Petilil. Another grass-type that's a bit more offensive than Cottonee.
- Skitty. A normal-type that evolves into another jack-of-all-trades, though one with more of an inclination to Special Attack moves.
- Delcatty (rustling grass). The evolution of Skitty. Waste a Moonstone? Pfft!
- Lilligant (rustling grass). Bypass those early years!
The Audino alone make this a great place to visit, and the many other pokémon more so. If you feel you're behind on levels - perhaps you struggled to help Hugh during the Sewers - then this is your go-to place for charging up your pokemon.