The title of this painting is Unconscious Rivals, implying a narrative content that isn't really apparent in the work itself but suggesting how Alma-Tadema's imagination worked. He wanted to present the ancient world as brand new, almost photographically convincing in visual terms, and to people it with humans exactly like ourselves, as opposed to classical emblems of virtue or vice. In this he was following the classical style more closely than some of his neo-classical peers in 19th-Century art. Even when Greek sculptors in antiquity were depicting mythological beings, they always endowed them with an essential humanity just as vital as their symbolic personae.
The play of light in this painting is magical yet perfectly naturalistic, and I love the way Alma-Tadema has obscured our view of the distant sea, which only makes us look deeper into the space of the painting to register it. It also makes us imagine walking up to the railing for a better view -- drawing us into the foreground space as we imagine navigating it.