Another way of getting the job done, without using tires-if that becomes a problem, is to excavate an area about the size you need, square off the sides, and build up your walls inside. You could have platforms above. Make sure to allow for drainage as well. The excavated hole wouldn't have to be too deep, because you would be using the fill to build up berms outside. Plant weeds and grass to keep the dirt in place. Just a couple of ideas. Good luck! It sounds neat.
