From the outside, Byzantine churches are homely brick piles. The exterior surfaces of the architecture diminish in importance: the patterning of brick and stone supplant pseudo-classical elements, although these sometimes remain in a vestigial form.
It has been noted that while for the Greeks, architecture had primarily been a plastic art of exterior form, allied to sculpture, for the Romans it was the interior that was defined plastically. Although this distinction is already clear in Nero’s Domus Aurea, it reaches its peak in the Hadrianic interiors of the Pantheon and the Villa Adriana. Apollodorus has been criticised for not really resolving the collision of the circular drum of the Pantheon with the rectangular portico – certainly from the outside, the meeting is unlovely, and modest attention is paid to the exterior surface of the drum. But this is because the Pantheon is primarily (I should probably stop short of saying exclusively) an interior.
Byzantine space is an extension of this tendency. The exterior ceases to be a site of attention, but as the exterior becomes more and more prosaic, the interior becomes deeper, richer, and more ornate. The interior surfaces become particularly lavish. The plastically-defined volumes of Roman interiors become vivid spatial envelopes enclosed by a precious gold skin.
The bodies of the saints, angels, and courtiers who populate these envelopes hover over a gold mosaic ground. Their space is not defined by aedicules or contextual clues. They live over luminous gold depths, emphasising their detachment from things of this world. Candlelit, these surfaces are animate: glinting and shimmering amongst regions of deep shadow. The circular haloes around their heads were sometimes slightly dished, inducing an atmospheric disturbance in this divine ether as the light reflects off the curved surface, and making the head emerge from the plane of the wall.
The mosaic circles of the floor of S. Vitale in Ravenna, c547, are carefully given drop shadows, producing the effect that the floor is actually a series of layered geometries. The floor, like the walls visually dissolves and shimmers.
In S. Vitale the space is layered so that interior views ever yield a complete picture. We always look through into new depths. From almost any point on the plan, it is possible to see through two, three, or more layers. Although the chapel is circular, a spatial form typically associated with unity and visual completeness, it packs huge depths into its plan. There is a sense that the space is larger on the inside than it is on the outside. Later churches amplified this effect of the infinitely-extended interior: St Sophia in Kiev, c1040, is an extreme example: the central and transverse axes and dome devolve into a field of piers and cupola.
From the perspective of an architectural culture apparently convinced that exterior form is the primary rhetorical dimension of architecture, Byzantine churches appear unimpressive. But the spatiality of the interior and the calibration of diffusive effects are remarkable.