Safeguard

Long ago, when the Skytower stood on the ground, there were ancient sentries that guarded the roads. They burned uninvited guests with lightning!
The spirits have long left their stones, but I still hope to return them. Their lightnings were so good!
I’m going to give you a device I recently discovered. I think Skrakan used it to turn the stones on and off. I tried to use it myself, but failed, I’m not good at magic, and my ears are rather old to hear things. Try it, will you?

Have you succeeded?

This is perfect! No enemy will enter town now!