Cadsuane said it would take someone extremely powerful and skilled to overwhelm her wards. We also know that neither saidar nor saidin could be detected around the Aes Sedai who'd been murdered or put in the trance, which means the True Power. Moridin and Shadar Haran arrived, plundered the room, took care of the Aes Sedai and Elza and freed Semirhage. Moridin left before Semirhage was freed because she would sense a trap if he told her to go for Rand with the male a'dam. However, if SH told her, it would seem like permission from the Dark One to satisfy her lusts, and she'd be easily duped, like she was. Moridin did the same with Graendal - he appealed to her lusts by asking her to destroy Rand's mind. It also led to her downfall. He's going to move on to Demandred next, who is very easy to bait.