Plutarch, Romulus
Bernadotte Perrin, Ed.

[5] And Tarpeius also was convicted of treason when prosecuted by Romulus, as, according to Juba, Sulpicius Galba relates. Of those who write differently about Tarpeia, they are worthy of no belief at all who say that she was a daughter of Tatius, the leader of the Sabines, and was living with Romulus under compulsion, and acted and suffered as she did, at her father's behest; of these, Antigonus is one. And Simylus the poet is altogether absurd in supposing that Tarpeia betrayed the Capitol, not to the Sabines, but to the Gauls, because she had fallen in love with their king. These are his words:—

And Tarpeia, who dwelt hard by the Capitolian steep,
Became the destroyer of the walls of Rome;
She longed to be the wedded wife of the Gallic chieftain,
And betrayed the homes of her fathers.

And a little after, speaking of her death:—

Her the Boni and the myriad tribes of Gauls
Did not, exulting, cast amid the currents of the Po;
But hurled the shields from their belligerent arms
Upon the hateful maid, and made their ornament her doom.