They made the base incredibly hollow
It still is VERY heavy but is just light enough so that it works

That wouldn't hold that much weight up though

The line things poking out of it are like bird wings just made of metal flaping 10 times a second to keep it moving
This might seem like more trouble then it's worth and you would be right. I have no idea why they made this

How would a flying city work
Just because it's 2,000 years in the future doesn't mean the laws of gravity change right??