Milan’s layout, with streets either radiating from the Duomo or circling it, reveals that the Duomo occupies what was the most central site in Roman Mediolanum, that of the public basilica facing the forum. The first cathedral, the “new basilica” (basilica nova) dedicated to St Thecla, was completed by 355. It seems to share, on a slightly smaller scale, the plan of the contemporaneous church recently rediscovered beneath Tower Hill in London. An adjoining basilica was erected in 836. The old octagonal baptistery, the Battistero Paleocristiano, dates to 335 and still can be visited under the Cathedral. When a fire damaged the cathedral and basilica in 1075, they were rebuilt as the Duomo.
source : wikipedia
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