Each card has a cost, noted in the top right corner; Snake can only play so many cards per turn, and the cost of those cards determines how much time passes between turns. Moving costs almost nothing, but equipping a weapon and firing can eat up a lot. So the game becomes a balancing act between getting into position to evade or destroy a guard and calculating exactly what cards to play to do it all safely.
Though Snake doesn't move in real-time, most of the classic Metal Gear actions are still around as options. He can crouch and crawl, flatten against walls and make sounds to attract enemies. There's no neck-snapping, though. Violence has to be through ranged weapons or knocking a guard down, which is just a setup to use a ranged weapon. That's a disappointment; even in the strategy gameplay there should be room for Snake's most famous moves.
Some of the card system's complexity might be meant to conceal the fact that the computer's artificial intelligence isn't much to write home about. The series has featured better and better artificial smarts over the years, but here the genius is on break. Enemies are aggressive, but none too sharp, and there's little unit thinking in evidence. Guards behave like disconnected individuals. While the game does get difficult, the challenge lies more in learning to use cards than overcoming the AI.