Re no guards, I’m guessing it’s a combination of

• Ser Criston Cole putting guards in strategic locations to ensure he isn’t caught.

• Larys Strong eliminating a handful of hand maids (and similar folk), replaced with his own. These new folks might not have the family in mind.

• Kingsguard hanging out with King Aegon.

• Overall not expecting an attack like this. Kings Landing is confident if attacked it will be via dragon.