The basement is incredibly unsafe due to the accumulation of unsanitary things (mouse poop) and pointy/collapsey things (general garbage). But it's still technically part of the premises of the business, and Brun is technically a customer on the premises of the business. If she suffers any injury from the basement trip, Dora is liable as the business owner. The waiver exempts Dora from any responsibility if a pile of giant T-rex statues falls on Brun, or if a spider eats her.