Devil Castle and the other four castles have a shooting range of one hundred meters, a meticulously designed feature. Ordinary Treasure Stealers wouldn't possibly bring a sniper rifle for a heist—it's too much trouble—so they usually carry convenient handguns.
Hence, the snipers at Devil Castle were very safe. They could wantonly shoot at people inside the castle, while the Treasure Stealers could do nothing about it.
However, this was true only for ordinary Treasure Stealers; for Wushuang, it was completely useless.
