After LiGHTs sans Tiara are caught infiltrating the castle, Eliza attacks Lynette and Ashley. When a panicked Lynette tells her that she's a huge fan of Ray and Eliza, Eliza doesn't react because she already knew this from stalking their unit after Tiara joined.
The royal palace in Bristol seems incredibly easy to infiltrate and dodge the security of until you realize that Rosetta has been living and serving there since childhood, and has been sneaking in and out of it with Tiara for years. Of course she'd know how to dodge the guard's patrols. She likely knows the entire layout by heart, including the secret passages they used.
