When a playwright attempts to write action, the result can be unpredictable, as can be the viewing experience, which is exactly what happened to me with Spartan. I loved everything about the first half, but after the "twist," everything goes off the rails and my levels of engagement decreased monumentally. Had Mamet stuck with a more conventional narrative, I believe this could have been truly great.
If this had been made about 30-35 years earlier, and the director had been John Frankenhimer or Fred Zimmerman or someone of that caliber, we would be talking about this film as one of the greats of its genre. This is a movie that gives away nothing and is not encumbered by exposition in the least. It's almost as if it's geared for a viewer not made of this "just add water" era.
How to put together the framework of a thriller and themes that haunt every classic Greek tragedies like Exile and Disguise (Did you notice how many times Val Kilmer changes clothes here) for instance, that's the lesson David Mamet teaches us in Spartan. Sometimes it is working, sometimes it isn't. Recommended though.