Originally Posted by piemelk
they gained a foothold on Cadia but their fleet has been devestated ?
No, if memory serves most of the chaos fleets are largely intact. However when Abaddon lost the black fortresses (Talismans of Vaul) he did pull his forces back into the eye.
He had managed to gain a foothold on Cadia, one of the most heavily defended planets in the Imperium; an act that no other chaos warlord (other than himself) has actually accomplished.
Even without Cadia under the control of chaos, countless other worlds were lost, meaning the grasp held by Imperial forces on the Cadian gate is weakened. This in turn will allow chaos ships greater opportunity to escape the eye and cause havoc.
Foothold on cadia...can't the Imperial Navy just bomb/lance them into oblivion?
Or cant cant imperials just land some siege engine of doom and macroshell them back into the warp?
No, because much of the fighting would have been taken to the various Kasr's. These are the city-states, major population centers, of Cadia and a fair majority of that is within urban sprawl or underground. The Kasr's are also fortresses, designed to withstand siege from the likes of orbital or ground forces, meaning that bombardment is useless and a direct siege will be resource consuming.
The thing about Cadia is that its greatest strength, that it is a fortress world, is also its greatest weakness. It is designed to be near impossible for someone to succeed in attacking, but if someone does than Imperial relief then has to go up against those same defenses, and any damage they inflict means that they have less to work with when they retake locations.