Meaning of "step in"

English as a Second Language English Vocabulary, Grammar and Idioms
1

Deat Teachers,

Can you expalin to me what “step in” means in this sentence?

“She made her way across the brow of the mount, to where the bridge, stepping in Gothic arches on top of a Roman aqueduct, began its lofty walk from the castle.”

Is it saying that the bridge connects to a Roman aqueduct?

2

I think it means that the Gothic arches rose in a stair-step pattern, or just that there were rising tiers of Gothic arches.