I dug up a copy of Salamandastron and noticed something about the cover:
On the U.S. PB cover is:
In the Tradition of Watership Down
I know the definition of "in the tradition of"
having characteristics similar to a particular person or thing
Now, I've dug through every single cover of every single Redwall book*. I'm 99% percent sure that this line doesn't show up on any over cover in this series. I'm also 99% sure that this is the only reference to a non-Redwall book on a Redwall cover; the only other references I saw on covers are to other Redwall books.
Given that, I assume that "In the Tradition of Watership Down" is meant to specifically be for Salamandastron. I'm just not sure how. It can't just be the talking animals (though that's definitely a part of it), because that would apply to every single Redwall book. Another curious part of this cover is it's on a US edition; I'd expect UK readers to know more about Watership Down than us ignorant Americans.
So, how can Salamandastron be considered "In the Tradition of Watership Down"? What elements of plot/setting/characters combine to make the works similar?
* Every single cover that's on the Redwall wiki, which is in general the US and UK covers only, and often just the front cover.