Whenever an instant or sorcery spell is cast during your turn, Sentinel Tower deals damage to any target equal to 1 plus the number of instant and sorcery spells cast before that spell this turn.
You choose the target for Sentinel Towerâ€™s ability, regardless of who cast the spell that caused it to trigger.
Sentinel Towerâ€™s ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
Sentinel Towerâ€™s ability counts all instant and sorcery spells cast in the turn before the spell that caused it to trigger, even if those spells are still on the stack or have been countered.
For example, the first instant or sorcery spell cast during your turn causes Sentinel Tower to deal 1 damage; the second causes it to deal 2 damage, and so on.