I am currently on the "build a trap fence" part of the advanced game course. I have built, what I think, to be an incredibly long fence. When zoomed out to the travel map, the fence is five squares long.
I started by using just trap pits (staked and covered), but when that didn't work, I also put in big and small traps.

Does anyone know the minimum requirements for a trap fence to complete on a game course?

I had a problem with this too. I eventually solved it by doing -

Fence - Fence - Pit - Fence - Pit - Fence

Win.

No idea if it works for everyone of course =)

Woooooow, it doesn't have to be that long...

If there are, say, 25 tiles of fence and, say, three covered trap-pits it should be okay.

I had trouble with this the first time around as well. The problem (I think, anyway) is that the game is looking for a trap fence with stakeless pits. I was adding stakes as I went and the task never completed, but when I built a second fence, about twelve tiles long with three regular pits along its length, the task triggered.

Ah, that's it! I went to my fence and added three covered pits without stakes, and it completed. Thanks!