Newport Bridge has stood on approximately the same site (next to Newport Castle) since a bridge was first built, probably in the twelfth century. The original bridge would have been built largely of timber – there was no proper stone bridge until 1800 AD, when William Coxe noted it was under construction.
Before the bridge was built there may have been a ferry crossing - but the depth of tidal mud almost certainly precludes the possibility of there having ever been a ford. The lowest point down river where it is now possible to ford the river at low tide with any degree of safety is to the north of Caerleon Bridge.
Apart from the discovery of late Roman coins found on the site of the castle there is no evidence that the main Roman road across South Wales (later referred to as the Julia Strata) ever crossed the river at the site of Newport Bridge, and the obvious route was for this road to cross the river by the bridge at Caerleon.
The river level and the river course have fluctuated over the centuries, but there is a narrowing of the Usk valley at Newport Bridge, so that the river would have always remained fixed at this point. When a bridge was built at Newport it would have been a better route to travel across South Wales than to use the ford or bridge at Caerleon.
To the east of Newport Bridge a ridge of land from the direction of Christchurch village slopes down to the river. A Roman road from Caerwent to Caerleon would have left this ridge on the east side of Christchurch. After the building of Newport Bridge in the Middle Ages the road through Christchurch would have been an important route across South Wales between Chepstow, Newport and on to Cardiff and the west. To the west of Newport Bridge the land gradually rises along Newport High Street, and rises more steeply up Stow Hill.
Possibly the earliest record relating to Newport Bridge is in a grant of 20 acres near the bridge of Novi Burgi (Newport) and near the River Usk, to the monastery of St Peter at Gloucester. This grant was made sometime between 1072 and 1104 AD.  There is also a reference to a bridge at Newport in 1185 AD when the Pipe Rolls (the annual audited accounts of the king) have a reference to repairs of a bridge at Novi Burgi. After that there are a number of references to the bridge, and in 1418 AD Bishop Edmund Lacy of Hereford was offering an indulgence (i.e. remission of punishment due for sin) for anyone who contributed towards the repair of the fabric of the bridge. 
In 1486/7 there was a ferry across the river whilst a new bridge was being built and John Leland mentions the wooden bridge over the river sometime after 1533. An Act of Parliament in 1580/81 authorised repairs ‘to a greate Bridge of Tymber called Newporte Bridge’ and ‘is of late fallen to greate ruyn and decay and likely dayly (not repayred) to become not passable’. 
Newport Bridge was the key to controlling traffic up the river and travelling through south Wales. It would have limited the size of ships going to Caerleon – ultimately ensuring Newport’s pre-eminence in the area.
1. Reeves, A.C. (1979). 113
Bob Trett 2010