Yipee thinks your cat - a giant litter tray!
Cats foul to establish territory, and they will re-mark it regularly.
First, find any traces of urine or faces and remove them. Scrub the area with boiling water and strong disinfectant. Rinse and repeat. This will remove the scent and break the 're-marking' circle for your cat. Then scatter some moth balls over the area - they last a few weeks, and can soon be preplaced. Your cat's delicate nose won;t go near them, and she will hopefully mark elsewhere in her territory.