Even though the mercenaries were still suspicious and curious about Rhode's orders, they, including Marlene and Lize, obeyed and left shortly after. Of course, it wasn't that Rhode didn't want them to know his secret. After all, he only needed half a day's time to complete building this humongous Fortress. Unless one's an idiot, if not, no one wouldn't think there's a problem with this. However, the fact that Rhode was willing to build the Fortress in front of them proved that they were his trustworthy confidants. As for why he made them leave, partially was for the element of surprise and also for safety reasons. After all, only Rhode had access to this construction system and who knew if the system might treat them as obstacles and kill them all if they lingered around. Furthermore, rebuilding the Fortress wasn't as simple as touching up the previous stronghold, so it's still best to avoid any possible dangers.