I free longe in a not-very-round pen, but it seems to work! Yes, I use a whip - a longing whip - and sometimes I will even have to use the butt end of the whip! It is not always pretty when done at our place, but the area is large enough to not hurt young legs (at least we hope it is), but they do get worked, mostly for conditioning. We have one (Rusty) who really hates going counterclockwise, so he is a real struggle. We also do 3-4 at a time and hope the younger ones learn from the older ones. Some are just easier than others. Our yearling colt that was just gelded last week was worked by himself yesterday, for (I think) the first time, and he got the concept immediately. Kept moving, changed direction when I asked and even trotted and cantered on voice command with reminder from whip. I will also admit that sometimes we let the horses decide when to change direction, again, as long as they do change direction and keep up a nice usually) trotting pace.
About the whip - we notice the 4-Hers love to crack it, but I usually just hold it out to keep them moving in the right direction, making sure not to get it in front of them. I will also circle it over my head to keep them going. Also, if you ever plan to do liberty, when you stop the work is the perfect time to practice your catch! Of course you can use treats for practice but not in the class.