In ancient Rome, the Forum was the center for rituals and civic and political events. Also, because there were citizens of all levels present for these events, the forum was the heartbeat of Roman life. Thus, in Wharton's story, "Roman Fever," the forum symbolizes activity that is sensual and wildly passionate in nature, as well as being a site of victory and defeat. 

For Mrs. Ansley, the coliseum symbolizes a delicious moment of unbridled passion in her youth, a moment whose memory she has long cherished. This place in Rome is also where she defeated her rival, having made passionate love with the other woman's fiancé and later given secret birth to his child.
It is, indeed, ironic that the two women who have "visualized each other...through the wrong end of her little telescope" should bring their daughters to this very place where they once were on holiday together and, at last, have an intimate conversation which reveals the truth of what happened in the Forum. For, Mrs. Slade learns that her friend Grace Ansley was not the victim of her cruel deception of a forged love letter. Rather, it has always been she who has been deceived, thinking that the "illness"--the Roman fever--from which Grace suffered for so long was caused by the damp air of the Coliseum when it was no illness at all, but a pregnancy which had to be disguised. So, now as the two Victorian matrons overlook the ruins of a great empire where competitors engaged with one another long ago, a supreme victory of the soul is won.