The progression is similar to Underfoot style, complete so many group missions to gain access to the upper tiers. So basically this page points out the missions you need to complete in order to progress through the expansion I will be adding what you need in order to request the missions on this page soon, but for now see the quest links. Each of these are group missions similar to Underfoot. Tier 1 and 2 are open in HoT, so you can explore them freely.
Click "show" to view the requirements to request each mission.
Must complete the achievement It's Dangerous to Go Alone, in order to access any tier 3 zones.
Must complete the achievement Have Fun Storming the Castle, in order to access any tier 4 zones.