This is the first Kingdom movie I saw in the series, but after being so impressed by it I will now go start and watch the first, second and third.
I thought this was just going to be your average "flashy war" movie. I turned out to be very wrong.
Why should you watch it?
The direction is top-notch. The cinematography and soundtrack is perfect. The acting is excellent. The story progression and character development is equally excellent (something I thought even though I haven't even seen the predecessor movies).
The best type of movie is one that keeps the suspension going, and that makes you care about what is happening and to whom. This is one of those movies. Do yourself a favour and watch it.
The only (very small) minus I can think of is that some characters, and action scenes, were a bit "over the top" and bordering cheesy.