DIY treehouse safety - what did you do?

Scrolling through and saw the treehouse post and got inspired but also paranoid. Before we even think about building anything, what safety measures did people actually put in place? Railings? Harnesses? How high is reasonable for kids? My 8yo is pretty fearless which is both awesome and terrifying for a parent lol. Don't want to be the dad who built an injury machine in the backyard but also don't want to build something that's no fun. What's the balance?