The answer
In addition to the above-mentioned practice of open-air baptisms, baptisteries were also used from an early stage, as soon as Christians started to build churches or at least to dedicate certain buildings for worship. Some baptisteries were discovered in the catacombs of Rome, where worship was conducted under persecution. Thus, in brief, we can conclude that early Christians began to assign special rooms for baptisteries about the same time as they allocated certain houses for worship.
Fr. Tadros Yacoub Malaty