The Sabini are attacking Rome, and have found Tarpeia, a Roman maiden, outside the walls.
Titus Tatius, Sabīnōrum dux, Tarpēiae optiōnem mūneris dedit, sī exercitum suum in Capitōlium perdūxisset. Illa petiit, quod Sabīnī in sinistrīs manibus gererent, vidēlicet aureōs ānulōs et armillās. Quibus dolōsē prōmissīs, Tarpēia Sabīnōs in arcem perdūxit, ubi Tatius scūtīs eam obruī iussit ; nam et ea in laevīs habuerant. Sīc impia prōditiō celerī poenā vindicāta est.
Titus Tatius, the Sabini's leader, gave Tarpeia a choice of gifts, if she would lead his army into the Capitol. She asked for what the Sabini were wearing on their left hands, namely gold rings and bracelets. Titus deceitfully promised these, and she led the army into the citadel, where Titus ordered that she be buried under shields, because already they'd had these on their left arms. In such a way treacherous betrayal is revenged by a swift punishment.
Why is habuerant plural? I thought it was just Tarpeia, and it should be nam et ea in laevā habuerat "because she'd already had these on her left hand.". What, then is the subject of habuerant? What have I missed?