new Corsairs' Stronghold battleground won't be playable until July 1st, but it's coming in the midst of such a massive patch that En Masse
is making it available for you to preload. There's five gigabytes worth of stuff to install, too, so the sooner the better.
The update post
notes that the patch will not preload while you play, so you'll need to leave the launcher open for a bit.