Are there humans in the world of Redwall?

No there aren't.

According the FAQ on Brian Jacques' official Redwall website;

Q: I have just started your books. I have read Redwall and am now working on Mattimeo. At the beginning of Redwall there is the "riderless horsecart" and now in Mattimeo there is mention of a ship that sank. Are there humans somewhere in the realm of Redwall?

BJ: No, there are no humans, my first book Redwall did mention the horse and cart but no humans are ever in the stories and I don't intend that they should ever be. The ships are generally crewed by vermin.

In the original book (Redwall), it is implied that yes, there are humans (the horse and cart). However, Jacques states in an interview that there aren't humans, and later books have no mention.

I suspect that in the first novel that Jacques hadn't fully developed the world (as he didn't expect it to be published), and that he may have intended for there to be humans.

I believe that Redwall was the first book published in 1986. Cluny arrives in a large hay cart pulled by a horse. In dialogue later on the elders recall from old records accounts of Cluny's hoard stampeding cattle through villages. Only conjecture on my part, but I think the series evolved quickly to exclude humans all together.

It's kind of a tricky question. It seems as though Brian Jacques intended humans to be in the Redwall world in the first book (the horse and cart etc.), but quickly changed his mind. However, there are hints that a human world lies somewhere far far beyond the sea. Some pieces of evidence were when the creatures of Salamandastron found what they called a "coin" on the beach (Sable Queen book) and when the wildcat leader in High Rulain says that the other cats are feral, and used to be "tame" and serve "stronger beasts" across the seas, but were liberated by his kind long ago.

there are no humans mentiond in the books as far as i can tell. the cart in the first book could have been build by cluny and the horse could have been captured. but i did not realize that the cats said that they were tamed by stronger beings across the sea. and i dont remember the coin but it is possible that there are humansin redwall.