Here's also the late Reformed theologian and historian Philip Schaff describing early church interiors:
"
In the middle of the sanctuary stood the altar, generally a table, or sometimes a chest with a lid; at first of wood, then, after the beginning of the sixth century, of stone or marble, or even of silver and gold, with a wall behind it, and an overshadowing, dome-shaped canopy, above which a cross was usually fixed. The altar was hollow, and served as the receptacle for the relics of the martyrs; it was placed, where this was possible, exactly over the grave of a martyr, probably with reference to the passage in the Revelation: 'I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held.' Often a subterranean chapel or crypt was built under the church, in order to have the church exactly upon the burial place of the saint, and at the same time to keep alive the memory of the primitive worship in underground vaults in the times of persecution.
The altar held therefore the twofold office of a tomb (though at the same time the monument of a new, higher life), and a place of sacrifice.
It was manifestly the most holy place in the entire church, to which everything else had regard; whereas in Protestantism the pulpit and the word of God come into the foreground, and altar and sacrament stand back. Hence the altar was adorned also in the richest manner with costly cloths, with the cross, or at a later period the crucifix, with burning tapers, symbolical of Christ the light of the world, and previously consecrated for ecclesiastical use, with a splendid copy of the Holy Scriptures, or the mass-book, but above all with the tabernacle, or little house for preserving the consecrated host, on which in the middle ages the German stone-cutters and sculptors displayed wonderful art." - Philip Schaff,
History of the Christian Church, Volume III: Nicene and Post-Nicene Christianity. A.D. 311-600;
Section 105: Interior Arrangement of Churches