All of that is in 5E and it is detailed in Waterdeep Dragonheist. The walking statues are stated at CR 18 and they are gargantuan constructs. Waterdeep has a dragonward, dragon species literally can't enter the city unless given permission by the wielder of the Dragonstaff of Ahghairon. In addition to the constructs and the multiple arch mages, the leader of the emerald enclave who is a literal demi-god resides in Waterdeep. Oh, and the open lord is a chosen of the goddess of magic, Mystra. She is a CR 17 arch-mage. I don't think an invasion would be possible in canon unless you had vast numbers helmed by entities of the level of an Acererak or Orcus.
