Illustrations of history

Just looked it up because I was wondering as well.

First, the image says "aqueduct bridge" for the first arched bridge, not "aqueduct", because the whole thing you see there is the aqueduct.

Second, while Romans also simply built walls if they didn't need as much elevation (and because they're easier to build than those arches), as soon as you have successive arches, you have an arcade. So in the image, they're both arcades and it's just labeled weirdly.