Shaula didn’t say the secret because they didn’t ask for it before. The first thing was that if anyone secretly leaved the tower, he/she would be killed, although Shaula didn’t want to do this. Next thing was that once the trial has started, the challenger cannot leave the tower, or Shaula will start killing everyone. The priority of this order is higher than Subaru’s “not hurting my friends” order (Shaula did the pose that is the same as Subaru’s pose, the one in which he points his finger to the sky). Other prohibited things include: resist against what the trial says, does something inappropriate to the library, and destroys the tower.