The Heavens were the home of many of the gods before the dragon declared their war, during the war, they became a battleground and then, in the aftermath, tombs. But not all of the servants and soldiers of the gods were slain, there was no need and that attack was so sudden and overwhelming that many were not able to mobilize rapidly enough to intervene. To prevent them from causing trouble, the links between the Heavens and the world was severed by the Empress but not completely . . .
While most of the portals between the world and the Heaven were deliberately destroyed, some were spared for artistic (or other) reasons and some survived as they were not recognized as pathways. Thus a variety of hidden gates, stairways and more unusual routes remained, most of them lost or unrecognized. The majority of the suviving portals are Moongates that require the light of the proper phase of the moon to open, though the right magic can trick them into opening as well.
Equally, there are ways to bring those servants of the heavens into the world, by variants of the summons more commonly used to bring diabolic beings from the dark hells. Many summoners consider the so-called angels more tractable servants, less likely to betray the summoner, but they are bound by other rules that make them less suitable for other tasks, they are terribly prone to issue warnings before attacking and show mercy, for example.
Occasionally -every century or so- someone, usually a dragon, will decide that making a raid on the heavens is a brilliant idea and will launch an expedition to do so. Most never return but those that do return are blooded and reduced in numbers, bearing back fantastic treasures and tales of horrific battles.
Notes: What better high level adventure than trying to plunder the vaults of heaven?