Battlegrounds are areas filled with monsters that only Warriors can battle. Each battleground has a level requirement. As warriors defeat monsters, they'll gain more experience and level up to gain access to stronger opponents.
Standard battlegrounds are always available to Warriors that fit the level requirements. Some of them will only appear in your battleground list if you're in a specific village.
These battlegrounds may never return for one reason or another. The two groups listed below had their enemies merged into the new standard battlegrounds: Mystic Gardens, Mystic Caves and Mystic Depths.