On the front range of the rockies--I live at 6400' elevation and do tree work at places from 6000' up to 8500', there's no such thing as level ground. A good portion of the trees I cut--mostly Ponderosa pine and some Doug Fir--have to be felled in places where no wheeled or tracked vehicle can access. I use a Portable Winch--this Canadian-made, Honda motor-powered capstan winch is a jewel. I couldn't do without it. It weighs 35 lbs. and is rated at 2200 lbs of pulling power. I anchor it to a tree and run a 200' length of 1/2" rigging rope to my work, uphill, downhill, no matter. I usually have one or two snatch-blocks set in trees to: 1. keep my rope off the ground, keep it clean, and reduce friction, and 2. as redirects, changing the angle of pull to avoid rock, other trees, go around a house, etc. The thing pulls at 40' a minute, which I thought would be slow when I bought it, but is just about right. Every now and then, for heavy logs, I put a block on the end of the log and double the rope back (one end of the rope needs to be anchored, and the other runs through the winch) to get double capacity from the winch at half the speed. For lighter stuff and bundles of slash, I rig two or three loads in each pull.
I'm not much of a phone or video guy, so nothing on hand to show it at work. If anyone's interested in seeing it, there's video at the Portable Winch . com site. As the thing is made in Quebec, the video I've seen there is pretty much flat land, northern hardwoods. To imagine how it works in my application, just turn your head 45 degrees as you watch.