Translation:There are many tall trees by that fence.
Did I already mention that English is a weird language? :)
"At" sounds a little off here, like the trees have some business to do with the fence. Or it's just much closer than "by". You can compare with German "am Zaun" (at the fence) and "beim Zaun" (by the fence).
Anyway, I think using "at" here should be acceptable, too.
I have no idea how English works. "I met him at the fence" sounds fine to me, but "There are many tall trees at that fence" sounds wrong. I can't explain this.
This is the reason why, when people ask me "How many languages do you speak, anyway?" I usually say "less than one" these days.