When constructing the glider, Sturm used coal to blacken the construction. Thanks to that and the dark lightning cloak, the short flight through the air went by unnoticed. With smoke filling the skies and burning warehouses everywhere, the citizens and guards had higher priorities than staring into the sky anyways.
The ride was shaky, but the construction did not fall apart over the 200 meters distance. Sturm just concentrated and held as still as possible to not deviate from the trajectory until he finally landed inside the church tower. If Sturm had not been a lot stronger than even the strongest people on Earth, then his momentum would have carried him straight into the giant church bell, alerting not only the guards dozens of meters below on the ground but every other available guard in a few kilometers radius,
