When you hit the 3 lines options in the top right, its over with Ruins, Glroy, and for even more xp and items the higher CP each character is, the higher they climb. You get like 10 to 15 million XP per character with a run. You may also log into these characters while they are in the Tower, it only pauses the advancement until logged back off. You may also not upgrade/requip/unequip any items on a character in an active Tower session.