The Watership Down Warren is the magnificent place that Fiver envisions, where Hazel, Fiver, Pipkin, Bigwig, Hawkbit, Dandelion, Blackberry, Silver, Acorn, Speedwell, and Buckthorn, settle after Sandleford Warren is gone. It is located under a beach tree on the hill of the same name. After it was founded, Hazel and his former hlessil, go out to find does to reproduce kittens so that they can build their new warren. They went to Efrafa to steal does from them but when they did The Efrafans followed them back to the Warren and started the Battle of Watership Down. Watership Down won the battle and Efrafa and Watership Down became close allies. Edit
In the film adaption, the warren was already made but was empty, allowing the rabbits to settle right in.
In the tv series, the bucks, Bigwig, Dandelion, and Hawkbit refuse to help Blackberry (who was re-imagined as a doe in the animated adaption) dig, since bucks don't dig. But after several events surrounding their first few days staying at the Down, the bucks lost to a game of bobstones, and they helped dig the warren.