The Battlefield Realm is a level 1000 place only, and you have to be in an allegiance to access it as the keeps that people capture in it are allegiance based keeps. You also need either Siege Weapons skill or Siege Magic skill. You also need to have completed all allegiance quests in all realms. The keeps are used to give you allegiance spell buffs such as buffs to Strength, Intelligence, Speed and Dexterity. You can get these bonuses by being in an allegiance and having the correct amount of keeps needed to cast such buff.
These buffs go up in a link, every ten levels you get, you`ll recieve a new allegiance buff, and it will be a +5 bonus. You can cast all 4 primary stat buffs (STR, INT, SPD, DEX) but can`t cast multiple ones (+5 INT, +10 INT). These buffs last up to an hour.
I`m not going to give away how many keeps you need for a certain buff, I`ll let you figure that out yourself.