In important ways the hurricane spared New Orleans: winds and rain were manageable. But the larger pattern of storm winds pushed massive amounts of water into channels and canals around the Crescent City.
Then the levees failed.
The breaching of the levees by high waters produced the flooding that devastated New Orleans.
Here's the Times-Picayune (Nola.com) interactive description of the disaster.
Along the Mississippi Gulf Coast the winds, rain and tidal surge of Katrina were cataclysmic. In Louisiana, the small city of Slidell was horribly stricken by high winds and storm-forced flooding.
Thus "Katrina" was a true monster.