Celia's Stupid Romhack: Vermilion City, the S.S. Anne and the third badge
What opens up at this point in the game, and everything you can catch by now that you couldn't before.
✓ Routes 5, 6 and 11, Diglett's Cave and the ship.
Celia's Stupid Romhack is built around working things out for yourself - what a Pokémon has become, what a move really does, which of the ten Pikachu you just met. The tables on this site are read straight out of the game's own data, so they hand you the answers before you have found them.
If you have not played it yet, the team's own advice is to go in blind, ideally with friends.
What opens up at this point in the game, and everything you can catch by now that you couldn't before.
