Sentinel Tower — ruling 1

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 — ruling 2

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

Sentinel Tower — ruling 3

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.

Sentinel Tower — ruling 4

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

Sentinel Tower : règles, fonctionnement et interactions (MTG)

Sentinel Tower is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever an instant or sorcery spell is cast during your turn, this artifact deals damage to any target equal to 1 plus the number of instant and sorcery spells cast before that spell this turn.

How it works : Sentinel Tower

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2018· Wizards of the Coast
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.
No other card is named in this ruling.
#2June 8, 2018· Wizards of the Coast
Sentinel Tower’s ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
No other card is named in this ruling.
#3June 8, 2018· Wizards of the Coast
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.
No other card is named in this ruling.
#4June 8, 2018· Wizards of the Coast
You choose the target for Sentinel Tower’s ability, regardless of who cast the spell that caused it to trigger.
No other card is named in this ruling.