"Rides a Dread Legion" and "At the Gates of Darkness" did have their good parts.
The Star Elves and investigation of the Demon Realms was all quite good IMHO. I found it all quite interesting and I love Amirantha as a character.
But the the final battle in "Rides a Dread Legion" really felt like an RPG scenario. I know Feist's books are based of their old RPG game but normally he manages to tell them in a way that doesn't feel like its an RPG world and it didn't work in that scenario. At the Gates of Darkness' only saving grace was the Star Elf brothers adventure and the one tiny twist Feist had at the very end. But both these books have a lot of information that helps you make sense of "A Kingdom Besieged" thanks to new Characters.
I do like Amirantha and Sandreena (Sorry if I'm getting these spellings wrong). Although I hated Sandreena's links to Amirantha and James. I thought they were very poorly played out and even unnecessary to a certain extent. It seemed like an attempt to get emotional with Sandreena but just ended up being too many interconnections between characters that neither came across as real or required. The relationship with the head of her order was nicely done though and Amirantha's introduction and deeds, together with his relationship to the warrior friend were all marvelous, particularly in "Rides a Dread Legion"
You can probably read "A Kingdom Besieged" without them, but I truly do like Amirantha and the Star Elves as they appeared in "Rides a Dread Legion" together with the Star Elves' adventure in "At the Gates of Darkness". It has a lot of information which will make much of what you read in "A Kingdom Besieged" make far more sense or simply deepen your knowledge of what they're addressing.
I normally have difficulty reading many Sci-fi fantasy novels, needing to go back and forth or re-read part because I've gotten lost in some of the information dumps and descriptions, but Feist really did make "A Kingdom Besieged" a VERY easy book to read. It was throughly enjoyable.
Be warned though, its by no means a stand-alone book. You'll be left on a slight cliff-hanger with many things still needing to be resolved in the next book.
P.S. seriously, if you get the chance, try reading just the prologue of the book in the store and you'll be left with little doubt that you need to buy it. Terrific prologues that hook you every time from Feist.