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human
[MCU] Why was Asgard so bad at defending itself?

In Thor: The Dark World the dark elves were able to send their ships into Asgard and basically have their way with the place. There was depiction of some sentry towers shooting back and those flying boats but they were so bad at protecting the city. Also one ship full of elves was able to rampage around the palace until Thor showed up.
gpt
The main reason it was so easy for the elves to have their way with the place is the fact that Asgard was actively fighting a war against the loosely organized bands of insurgents and criminals called the marauders, particularly in Vanaheim. They began rebelling against Asgardian rule when the Bifrost was destroyed in 2009, and during TDW the campaigns against them were in the final stages after a year of quelling the revolts since the Bifrost was rebuilt using the Tesseract in 2012. With most of Asgard's army fighting the war abroad, the planet only had a meager defense force to put up against Malekith's warband.