Detects stealth/invisible once per turn. Deals 1 damage to each attacker (that it can see).

Tower Rulings

The damage dealt by the tower is considered combat damage (so if it kills a Brave Knight that counts as "dying from combat damage"). It also means that the damage is dealt simultaneously with other combat damage.Sirlin, 03/14/16
If the attacker has swift strike, the tower still deals combat damage simultaneously as the swift strike. If the attacker has long-range, the tower still deals it damage to the attacker. If the attacker has flying, the tower still hits it as if it had anti-air.Sirlin, 03/14/16
The tower deals 1 damage to attackers who are attacking ANYTHING controlled by the player (or team in 2v2) with the tower. It doesn't just trigger when they attack the tower itself.Sirlin, 03/14/16
On an opponent's turn, your tower uses its detect ability the first time it can. For example, if the opponent attacks you with a stealth 1/1, the tower will automatically reveal it, and that attacker can't ignore your patrollers. If the opponent then attacks with an 8/8 stealth unit, the tower won't detect it because it already used its ability once that turn.Sirlin, 03/14/16
If an attacker has stealth or invisible AND another evasion ability such as unstoppable, the tower will still use its once-per-turn detect so that it can deal 1 damage to the attacker. This won't prevent the atatcker from using evasion abilities such as unstoppable though, so it can still ignore patrollers when it attacks.Sirlin, 03/19/16
If the tower already used its detect ability on an opponent's turn, then that opponent attacks with a stealth/invisible unit or hero, the tower won't hit that attacker. It won't even hit the attacker if it attacks the tower itself. In this example, the tower already used up its detect ability and it can't hit any more stealth/invisible attackers that turn.Sirlin, 03/14/16
When you reveal a stealth/invisible thing with your tower, that thing stays revealed the rest of the turn. It is legal for you to cast multiple spells that target that revealed thing later that turn.Sirlin, 03/14/16
In free-for-all, the tower will only hit an attacker that is attacking things you control. It won't hit an attacker that is attacking things controlled by one of your opponents.Sirlin, 03/14/16