Before the days of fancy Churches, this is what the inside of a Church looked like:
This is what the early Christians saw when going to church. In the absolute earliest days, the Altar and the rest of the church were separated by a huge curtain. The Antiochean Tradition preserves this practice to the present day. It took a century or two to develop the iconostasis and from then on the early Church had churches like this one. It is the Lower Church at the Fedorovskiy Cathedral in St. Petersburg (h/t to Jordanville Journal), which served as the "home parish" of the Russian Imperial Family. Actually, the iconostasis is a bit short (looks like an altar rail, which didn't develop until the renaissance; the wall was taller until then), but you can see the icons of the Mother of God and Christ are quite tall (a little ironic given that the iconostasis is short). Visible over the gate is the canopy over the Holy Table and the icon of the Mystical Supper (Last Supper) on the wall. Those two elements are very ancient and were more popular then than they are now. Above the steps is the choir for the clergy. Below the steps is the area for the congregation. Of course the ceiling is made to look like it's in some sort of catacombs, and appropriately enough this is the crypt level of a cathedral. By contrast, the Upper Church is in the baroque style, which typifies churches after the early Church period.