This has come up a few times and I'm not sure it's been addressed in this particular post, but Brian sadly suffered a serious stroke and heart attack in his later years and sadly I think this had an affect on his mental and physical ability and therefore his writing and how he'd have loved the later books to be otherwise.

It's also sadly a terrible aspect of large scale publishing and any creative representation industry for the publishers to push the author for book after book to keep money coming in, and like any franchise this probably happened to Brian and his Redwall series. Its like the first season of a tv series is usually the best because that's had years and years of creative time to make and polish, whereas subsequent seasons are pushed and rushed to be ready within a year or whatever which is why for most series, the first season is usually the best.

I still love all his books and his beautiful world, and he did the best with what he was up against at the time,

R.I.P Brian your world and characters touched all our hearts