So our neighbor's cat has been visiting us for a while, so I decided to build this ramp so that she could more easily get into and out of our yard. I'm not much of a woodworker, but was happy with how this turned out.
It's made almost entirely from cedar fence posts. Nearly 18 feet long (3 fence posts end to end with the dog ear tip cut off). Ripped cedar planks into 3 for a frame/support to prevent flexing. Posts are pine 2x4, and go 18 inches into the ground, which was the majority of the effort, since our ground is like cement...
That's very situational. If you're in a rural or semi-rural area that has small wildcats (or foxes or similar) already, adding a handful of domestic cats isn't going to disrupt anything much. The only reason to keep cats inside in such a place is for their own safety (from larger predators like coyotes, and from highway traffic).
If you're in Australia, Antarctica, or a protected island biome with no native small wildcats or canids, or you have a known endangered species in the area that cats are likely to prey upon, that changes the equation. If you're in a highly urban area, that changes things in a different way, because the danger to outdoor cats from traffic and other human activity rises exponentially.