To enter Briarheart the first time, you must prove yourself to the commander of the Tanglewood Fort in Nettleseed. There is a warpstone inside, so you only need to do this once.
There are no collection spawns in Briarheart. There is an Exploration collection, Briarwood - Briarheart. There is a Locked Treasure Chest, here, too. There is also no farm, but Rich Gold Mining can be found in plenty.
Briarheart is divided into two sections by a mountain range, with a tunnel connecting the two. The front half, which is where the back door of the Tanglewood Fort is. Here you will find the Croaking Vale, a Hard dungeon.
In the back half, at the "heart" of Briarheart, is Briarheart Palace, a World Tree like the one in Sanctuary, but a dark echo of that tree's brightness. Also, at the top of a switchback trail in the south, is the Briarheart Caverns, a Hard dungeon.
Briarheart has no notable quest series at this time.