When Nintendo realized how popular "Red", "Blue", and "Yellow" had become, they decided to support the production and release of more games. After "Yellow" came "Pokémon Gold and Silver", released in 2000, which had a total of 251 catchable Pokémon, as opposed to only 151 previously. A sequel to the two games was released, "Pokémon Crystal", which had several new features that were not on "Gold" and "Silver": Pokémon had animated sprites when they came into battle, there was a subplot within the games relating to the different letter shapes of the Pokémon Unown, and there was a new building called the Battle Tower, where players could battle very strong Pokémon to win items and set records. Once the player has beaten the Elite Four, they can go to Kanto, the region in "Red", "Blue", and "Yellow".