There is a storage chest in the master's chambers, across from the bed, on the left wall as you enter the room, where everything sent to the battle tower is stored (it starts off with a couple items in it, as well). You can transfer stuff back and forth after selecting it, similar to a merchant's inventory.
There is a key for that chest, but you will have do some exploring to get it (start with the alchemy platform). Hint:
Check out the table there.
BTW, I only noticed your edit by fluke.
