Sentinel Tower

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.

Sentinel Tower’s ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Sentinel Tower’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.

You choose the target for Sentinel Tower’s ability, regardless of who cast the spell that caused it to trigger.