The 9th Canadian Infantry Brigade had hoped to take Boulogne off the march as it advanced up the coast but it was stopped by the German fortifications 5 mi (8.0 km) from the city.

Despite the lower than expected level of material damage by the bombardments, the high degree of co-ordination between artillery, air force, tanks and infantry greatly aided the success of the operation.

In practice, Boulogne's landward defences were incomplete, many of its garrison troops were second-rate and demoralized by their isolation and the obvious inability of the Wehrmacht to rescue or support them.

[4] The city and port of Boulogne is sited at the mouth of the River Liane, which flows north-north-westwards into the sea, which is to the north-west of the centre.

He was ordered to create a substantial defensive zone but he had neither the specialist nor the resources to achieve this, "I merely put a big red circle on my map to show that the demolitions had been theoretically carried out".

The vulnerability of Boulogne was not appreciated by the Canadian Army commander, Harry Crerar, who judged that a full set-piece assault would be necessary, supported by heavy bombardments from land, air and sea and with specialized armour.

[6] He also wished to be certain of success, to maintain the momentum following the fall of Le Havre and keep up the psychological pressure upon the remaining fortresses at Calais, Dunkirk and elsewhere.

[8] The Canadians gained useful intelligence on the German defences through information from evacuated civilians (8,000 were expelled by the occupiers) and with the help of the local French Resistance.

This attempt at the destruction of defences was surprisingly ineffective; Heim said "amongst personnel, casualties were almost negligible" and that permanent installations suffered little damage.

The 8th Brigade captured Rupembert and its radar installation intact, and consolidated in Marlborough [1 mi (1.6 km) north-west of the city centre].

The 9th Brigade's early advance had been rapid but, once its defenders had recovered from the bombardments, they gave an effective defence with artillery and machine guns.

[14] The 9th Brigade's North Nova Scotias finally subdued Mont Lambert by 11:00, the loss of which General Heim believed "would make defence of the port impossible".

This area occupies a dominant position above the port and was (and still is) entirely surrounded by thick medieval masonry walls with a dry ditch in places.

As the Canadians prepared to assault with the AVREs, a French civilian disclosed a secret passage and a platoon was taken beneath the walls.

The Divisional reserve, the Cameron Highlanders of Ottawa successfully completed their assault on Herquelingue heights, south-east of the city, east of the river, overnight on 18/19 September.

A large German force had remained hidden in tunnels underneath the fortifications and had to be subdued separately on 20 September after making a nuisance of themselves (this was dubbed the bargain basement incident).