尖塔横の潜入者

For the ability to trigger, Spireside Infiltrator has to actually change from untapped to tapped. If an effect attempts to tap it, but it was already tapped at the time, this ability won't trigger.

This is a triggered ability, not an activated ability. It doesn't allow you to tap Spireside Infiltrator whenever you want; rather, you need some other way of tapping it, such as by attacking.