Q. What came before the font and the baptismal pool ?

Answer : One of these.  Its a 4th century Romano-British font-pool-baptistry thing.  Over a thousand miles to the North-West of Rome, a christian church situated in the port settlement of Richborough in Kent built this baptistry in their church located inside the walls of the Roman Fort.  It is big enough for one person to stand in, but not two.  It is floor or ground standing, and its shape and construction suggests that it was not much taller than this when complete, thus an adult standing in it would be thigh-deep in water were it to be completely filled.  However, it has no drain or signs of plumbing of the types associated with Roman buildings.  Which would suggest that it was either left filled, filled when needed, or not filled but used as a basin or recepticle for poured water.

Wall paintings from elsewhere in the empire show baptismal candidates having water poured over them while standing in a large bowl or hollow structure, and given its modest size and dimensions,  that would be a plausible way of using the Richborough Baptistry.  If that is the way it was used, it would add weight to the suggestion that mandating full immersion is a later innovation added to the basic act of baptism, and not an idea felt necessary by those nearer to the New Testament than we.

Rome in Wiltshire

The design of Bowood House in Wiltshire is based on the 4th century Palace of Diocletian in modern Split, Croatia. Robert Adam, the architect of Bowood, had surveyed and publicised the Roman Emperor's 'retirement' palace, which during the centuries before his survey had effectively become lost. Adam recorded the survey in his book "Ruins of the palace of the Emperor Diocletian at Spalatro in Dalmatia", published in 1764. Today, inside the public entrance, one of Adam's elevations is on display to illustrate how closely he followed the original.

Of course, the modern Bowood House is considerably smaller than Diocletian's huge complex with its accommodation for a small garrison, its numerous servants and private access to the sea. Nevertheless, Bowood still give one a feeling for the elegance of Classical Architecture, albeit mediated through 18th century eyes.