The Mead Hall allows you to receive reinforcement troops. You can only have one Mead Hall.
Under the Reinforcements tab you see the amount of troops you currently host. Here you can also send troops back if you need to.
Under the Ready for Onslaught you see the amount of troops that have joined your forces for the onslaught.
Requirements: You can only upgrade the Mead Hall if you have at least one Stone Quarry already upgraded to the same level as you are trying to upgrade to (except for lower levels, see table). Example; If you want to upgrade Mead Hall to level 14 then you must have at least one Stone Quarry that is upgraded to level 14 or more.