they did bombard it, Horus bombarded it for weeks, with all te fleet he had, the bombardemnt was mooving the tectonic plates of terra around, moving around mountains ect (and IoM ships throw terraton (by some crazy high end calculations low petaton) levels of firepower around (that means each broadside is the equivelent of a terraton of tnt going off)), but he didn't get anywhere with it beacose of the theater shield over the palace, it was after the bombardmend he decided for a ground assaultthe_hessian said:Chaos could have bombarded the crap out of the Imperial palace and won. They could have opened a couple of warp gates and sent wave after wave of daemons to destroy it.
and the emperor along with other phsyker on terra were doing their best to prevent too many deamons to be summoned