Abilities gained within the Tower depend. If it's something he could have gained himself then it's possible for him to retain knowledge of how to do it. Seals and the like do not apply to real abilities, though, because all seals done/undone were done via fake powers. If someone is capable of sealing/unsealing things with their normal powers though it could be replicated.

"Large portion of the Tower" would refer to multiple floors or very large groups of people/monsters. Half a floor would be alright.