Playcentre from scratch

[Adam1987]

Hi i am looking at building a playcentre from scratch 10 foot high 12 foot by 6foot will have one tower 5 foot by 6 foot a wooden bridge (with monkey bars underneath) 5 foot by 2 foot and another tower 2 foot by 6 foot there will be a 4 x 4 beam going across 2 foot back from the front for 2 swings, wondering what the best way would be to anchor it to the ground thanks