3. from Nantes to Pont-St-Martin

Tradition dates the first building back to the arrival of St. Clair in the 3rd century. He had brought from Rome a nail from the cross of Christ.
Archaeological excavations at the beginning of the 20th century identify a building from the 6th century. This cathedral was consecrated between 567 and 580.
Saint Columban probably prayed in this building during the days spent waiting for a ship to Ireland, which was too loaded to take to the sea and put the Irish saint and his companions on the shore who set out again towards the north of Gaul.
The Romanesque Cathedral led to the modification of the choir which dominates the 12th century crypt.
The present building was begun in the 15th century and the work lasted until the 17th century. During the Revolution the cathedral escaped destruction.
Violent bombings in 1944 led to a complete restoration of the building.
The nave shelters the tombs and the tombstones of Duke François II of Brittany and his wife Marguerite de Foix (parents of Anne of Brittany).
One can admire the cenotaph of General Louis Juchault de Lamoricière, his decisive military actions contributed to the rapid pacification of French Algeria in the 19th century, a monument erected in 1878.
A very beautiful exhibition of the history of the cathedral is presented in the crypt, accessible from the outside at the chevet level.
More details : Wikipedia

A la fin du 15ème siècle, le développement du château et de la cour ducale entraîne la construction, par les hauts dignitaires, d’hôtels à proximité. L’hôtel de la Psallette est ainsi édifié en 1462 par Jean Gougeul de Rouville qui y mène une vie publique et privée.

Au 19ème siècle, l’hôtel accueille la maîtrise de la cathédrale (la psallette) qui lui donne son nom.

Located in the historic heart of Nantes, the Castle of the Dukes of Brittany is the flagship monument of its urban heritage, along with St Peter's Cathedral.
On the city side, it is a defensive fortress whose 500-metre-long rampart walk is punctuated by seven towers linked by curtain walls founded in the 13th century.
On the courtyard side, it houses an elegant 15th century ducal residence made of tufa stone, in flamboyant Gothic style bearing the first signs of Renaissance inspiration, and other buildings dating from the 16th and 18th centuries. These contrast in their whiteness and sculpted refinement with the harshness of the exterior walls made of granite blocks separated by schist layers.
More info at: Wikipedia

The Via Columbani crosses the Saint-Felix lock.
The river, the Erdre, runs through the city centre of Nantes. A 700 m long tunnel leads into the port of Saint-Félix. This river tunnel was dug in the 1930s as part of the work to fill in the Erdre river to allow the Nantes-Brest canal to flow into the Loire.
The lock and landfill works were built at the same time as the tunnel in the early 1930s. This structure ensures the outlet of the Erdre into the Loire. It consists of a 50 m long by 7 m wide lock for the passage of boats, a removable weir and two bottom gates for regulating water levels.