Flak Tower is a passive card, which means that it activates by itself. But its success rate depends on the level of the card, meaning that it doesn't work 100% of the times. The success rate applies to each attack, not to the whole match 

Example: If the success rate is 60%, it doesn't mean that it will work for 60% of the total attacks of the match, it means that there's 60% probability of success on each attack.

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