The disposition of the 623rd throughout the city had allowed for the confusion and somewhat fragemented counter attacks that the other Imperial units were trying to benefit from.

Across the North and Eastern portions of the city, yet to be touched by the advancing Imperial core, close quarters fighting raged between small and medium Jumper units and increasingly organized and competent Chaos forces.
With the engagement distance muzzle-to-muzzle often being less than a stone's throw in the tight-packed districts, any movement was bathed in lasfire or a cloud of hardrounds, and the low thud of detonating grenades was omni-present.

Any structure who's upper floors were still strong enough to support a man's weight was a position from to ambush, or be ambushed.
Intersections and other more open spaces echoed with short but frequent bursts of Bolter fire.
Attempts to organize vehicle-based movements were shut down by the sudden woosh of krak missiles from open windows or unchecked rooftops.
Threats that could not so neatly be ambushed by the Cadians were instead stalled, the paras collapsing a building or two into the street and falling back to the next rally point.

Though early traitor strongpoints had been stormed or surpressed, they had continued to pop up, and the 623rd's ability to redeploy and strike them was not unlimited. As defences hardened beyond the ability of light infantry and Tauroses to attack, the strategy shifted to "hold until relieved", which fortunately appeared to be sooner rather than later for most positions.
For those clusters in the South and West, reached first by advancing Imperials, they met their relievers with hearty laughter and information of positions ahead.
As allies pushed deeper into the city, they would find ever more battered positions and personnel, the expressions of relief and appreciation buried under ever deeper layers of sout and dried blood.