I'm building a fort in the backyard, which will be 8 by 16 feet. I plan on attaching a zip line to the middle of the fort, and need to know how to stabalize the fort. If I assume a maximum allowed weight on the zip line of 250 pounds, and a minumum angle of decent of 10 degrees, the maximum force pulling on the fort would be 750 pounds, most of which would be directed horizontally. I am using 6 4 by 4's as the supports for the fort, and zip line would be attached 14 feet above the ground inbetween the middle two 4 by 4's. My question is how should I anchor the fort into the ground? My plan is to sink the 4 by 4's into the ground and possibly use cement as well. I willl also use small stakes, but how deep does everything need to be? I am also building a small attachment for the zip line at the other end - this will not be a fort, but I can not attach it to a tree. However, it also needs to be able to withstand the 750 pounds of force without toppling.