The Battle of the Seine was a battle between the Franks and the invading Norse fleet. The Franks, having build two forts at the moutyh of the Sene following Rollo's advice in order to block the Vikings' way to Paris, met the invading Norsemen with volleys of bolts that did great damage. When their longships managed to slip past the forts, Rollo and his men swung a huge chain that was hidden underwater and that caught the Viking's rear, while using catapults and flamed arrows to destroy the ships. The result was an astounding victory for the Franks and a severe defeat for the Danes, who lost hundreds of men and retreated back to their camp, which in the meantime had been devastated by a surprise Frankish attack.
In 804, King Ragnar Lothbrok, at the summit of his power, launched a colossal land and naval assault on the city of Paris, in West Frankia. Dreaming of the plunder, the riches and the fame it would give him and the Viking world, he assembled a fleet of over 100 ships carrying 2 000 warriors and attacked the town's remparts at first, only to be repelled with heavy casualties. The depleted Viking forces managed to enter the city thanks to a ruse from Ragnar and successfully raid and plunder it.
Ragnar Lothbrok decided to launch another assault on the city one year after his initial success. At Kattegat, a huge fleet between 100-150 ships is mustered, including ships from the kingdom of Vestfold-Rogland led by King Harald and his brother Halfdan, who seek fortune in the next raid following Ragnar's initial success. The fleet soon sails away, taking away, for the first time in their young life, the two brothers Hvitserk and Ubbe.
Meanwhile in Paris, Duke Rollo had married Princess Gisela and had been rewarded by Emperor Charles with the title of Duke. Rollo was to be alongside Count Odo the leader of the Frankish forces. Rollo's first advice to Odo was to build two forts on each sides on the River Seine in order to prevent the Vikings from sailinhg upriver and therefore reach paris. The two forts would hide a huge metal chain that would be used in order to knock over Viking longships. Rollo also advised Odo to begin the construction of a fleet of ships similar to that of the longships to meet the Vikings in battle and prevent them from reaching Paris' remparts.
The Viking fleet entered the mouth of the Seine and prepared to move towards the remparts of the town, only to be met by Rollo and his Frankish soldiers on their forts overlooking the river. As soon as the first Viking ships approached, Frankish crossbowmen began pouring down thousands of bolts on their opponents that found their mark or went crushing into the thick Danish shields. In the meantime, Ragnar had sent an ambush party of some 250 warriors led by Lagertha to hit the Frankish on their rearguard fort, only to be stuck in muddy swamps. Rollo discovered the feint and directed dozens of his crossbowmen towards the advancing party who was now the easy prey of the Frankish bolts. Many warriors and shieldmaidens were struck by the bolts but the party managed to reach a soft ground and form a shield wall. However, the huge metal chain erected on the advise of Rollo was raised, knocking over many longships. A huge chaos took place, while hundreds of Viking warriors fell, struck by crossbow bolts. Many others drowned or managed to reach the river side, only to be cut down by Frankish pikemen and crossbowmen. Lagertha's party, meanwhile, advanced towards the fort but Frankish crossbowmen took the Viking shield wall on the rear, taking out dozens of them and forcing the depleted force to retreat. The Viking fleet sailed away while the trapped ships and their crews were put to the torch with the use of catapults throwing flamed projectiles.
This first engagment was a conclusive win for the Franks, who had prevailed mostly thanks to Rollo's advice. They had sustained light if not few losses, and the forts had proved to be a strong asset, while the Vikings, on the other hand, had taken a heavy toll, with more than 400 dead and at least 20 ships burned. It appeared that this setback had ended the Norse invasion attempt. Furthermore, the viking encampment was attacked by Count Roland and dozens of horsemen, who burned tents as well as killing many warriors and women.