"Playsets" come in a hundred different variations. What's does your consist of? Are there manufacturer's installation directions?

We have ours installed over pea gravel. gives a decent cushion, and the kids have no issues running around barefoot on it. Drains well, doesn't get filled up with neighbor cat waste, and no slivers. Ours is a DIY platform with slide, and three swings.

It's a rainbow castle, the "rainbow" line. I believe its made out of redwood, and the support pieces in contact with the ground will be 4x4 redwood.

What does the manufacturer suggest?

These playsets are designed to be free standing, and do not need to be anchored into the earth. They weigh about 1600 lbs dry. They recommend a variety of surfaces, like pea gravel, sand, rubber mats, wood mulch, rubber mulch, and just grass. I have decided to build a dedicated pad out of landscape ties and fill it with rubber mulch. In my case I have two options:

1) I could just put it directly on top of the 6" layer of rubber mulch

2) I could brush back the rubber mulch back down to the weed barrier, and have the horizontal supports rest directly on that

I've read reports of these playsets having issues where the horizontal support pieces rot after a few years. But they never say if they are just placed directly on the bare grass, dirt, buried in mulch, or whatever. I'm thinking that if I put the horizontal support pieces directly on top of the rubber mulch, that would keep the wood the driest, and help prevent rot. But that might not support the playset as well or provide the best stability.

If I brush back the mulch and put the supports directly on the weed barrier that is over the dirt, I think that would be the best support/stability for the set, but I would loose 6" of height, and the 4x4 horizontal support beams would be buried in the rubber mulch, and not stay as dry. The bottom of the supports would touch the weed barrier and it would probably wick moisture from the dirt below.

I might be overthinking this too much, but these playsets cost $$$$ for what they are. I'm hoping to get at least 10 years of use out of it, and then have it be in good enough shape to sell and recoup some of my investment, or be an asset to my backyard when I sell my house.

In a way, I think you are "overthinking" it. 4X lumber is treated to withstand ground contact. Though it will not last forever, it's usually good for 20 years. That you are hoping for a mere 10, should have you well covered. Also, given that there are only 3 horizontal timber pieces in contact with the ground, should any decay before their time, they could be replaced as a relatively minor repair. BTW, whatever affects the horizontal members in ground contact, will also affect the bottom of the vertical legs. But I don't think this changes anything.

Since the manufacturer does not specify that they must be on a special foundation, I would set them on top of the mulch. Over time, the set will settle in some, but it shouldn't be of any consequence. If it turned out that play caused any part of the structure to leave the ground (such as a swing leg) I'd work on pinning it down with some kind of attachment after it is already in place.