From Bulbapedia, the community-driven Pokémon encyclopedia.
The Key System (Japanese: キーシステム Key System) is a feature of the Unova Link in Black 2 and White 2. It allows players to change the difficulty level of the game, switch between Black City and White Forest, and change the Mystery Door in the Underground Ruins. They can be shared between players using the Unova Link, but players can only share the keys that are exclusive to their version.
For the first time in a Pokémon game, there are different difficulty levels. There are three different modes: Easy Mode (Japanese: アシストモード Assist Mode), Normal Mode (Japanese: ノーマルモード Normal Mode), and Challenge Mode (Japanese: チャレンジモード Challenge Mode). Only Normal Mode is unlocked by default; Easy Mode and Challenge Mode must be unlocked through the course of the games.
Easy Mode decreases the player's opponents' artificial intelligence and the level of their Pokémon. As such, battles become easier. This mode is unlocked after defeating the Champion in Pokémon White 2.
Challenge Mode increases some opponents' artificial intelligence and the level of their Pokémon. Some opponents will also have an increased number of Pokémon, as well as held items. This mode is unlocked after defeating the Champion in Pokémon Black 2.
To unlock Easy Mode in Black 2 and Challenge Mode in White 2, the respective keys for each version needs to be transferred from a player with the opposite version via the Unova Link.
The version exclusive cities, Black City and White Forest, are found by default in Black Version 2 and White Version 2, respectively. They can be unlocked in the opposite game if certain conditions are met.
After completing five areas of Black Tower in Black 2, the player will be rewarded with the Tower Key. This can be transferred to White 2, which will unlock access to Black City. Likewise, after White 2 players have completed five areas of White Treehollow, the Treehollow Key can be transferred to Black 2. This unlocks White Forest for Black 2 players.
Players of either version can use the Unova Link to switch between cities once both are unlocked. However the cities cannot be switched if the player had saved their game within the current city.
A mysterious door in Clay Tunnel gives access to the Underground Ruins, where the legendary golems can be found, though only one golem will appear at a time, depending on which key is active. Regirock is the default Pokémon in both Black 2 and White 2, under the Rock Peak Key. Upon catching Regirock, and not just defeating it or obtaining it from another game, Black 2 players are rewarded with the Iron Key and White 2 players with the Iceberg Key; these allow access to Registeel and Regice, respectively.
While Registeel and Regice are exclusive to Black 2 and White 2 respectively, players of the opposite version games can obtain their final legendary golem by transferring the appropriate key through the Unova Link.