We put the washers on the card which was on the top. I was surprised it didn't tip over.
The changes we made are we took all the beams and columns off which left us with a card. Then we put three cylinders on the bottom that had another cylinder in each. We made the changes because it would tip over if we put a few washers on the top.
I learned that the smaller and more built they are, the more weight it held. For instance, my structure was about 6 inches tall and held 141+ washers. The tallest one held less than 5 washers.