Each time a Spell Card is activated, place 1 Spell Counter on this card when that Spell Card resolves. If a card with a Spell Counter(s) is destroyed, place its Spell Counters on this card. Once per turn, if you would activate a card effect by removing a Spell Counter(s) from your side of the field, you can remove that many Spell Counters from this card instead. If this card would be destroyed, you can remove 1 Spell Counter from this card instead.
Master Magician can be special summoned from your hand/graveyard using Citadel's spell counter. While building up the 6 counters can be somewhat slow, this can be used to revive Master Magician in a prolonged duel.
Once per turn, you can increase the Citadel's spell counter using this trap card. Can be a more guaranteed way to increase the number, but also useful if you use other monsters that can benefit from spell counters.