While we’re talking about Devil’s Bridges, Hamburg has one, too: It was built to make crossing the devil’s ford easier, so called because there were so many accidents there people couldn’t explain it otherwise. A carpenter was contracted, who is said to have made a pact with the devil, that the bridge may stay, at the price of the soul of the first living thing to cross the bridge. On inauguration day, then, the local reverend blessed the bridge and set off to cross it, when out of the bushes a rabbit appeared and sprinted across the bridge. A statute memorialises the occasion:
…the less exciting explanation is that back when Holstein was still under Danish rule there were two bridges close together, and the double bridge then turned into the devil’s one. dövelt -> Düvel in Low Saxon makes a lot of sense but is boring.
Slaves nowadays usually refers to either American-style chattel slavery or Roman-style slavery, both of which were systems much different than Serfdom.
But yes serfs will likely have built it, or were involved with the build under direction of hired stone masons, on order of a noble and with resources a noble paid for, under the general societal rule that serfs were to spend a certain amount of days a year working on infrastructure stuff as part of their taxes. For more details you’d have to dig into the law at that time at that particular place but those kinds of arrangements were incredibly common in Europe in the middle ages. You tended to be able to buy yourself out of having to work, and also pay your way in silver or gold out of paying in grain, livestock, etc.