While you probably know the TG-16 for it's shoot 'em ups, this is one of the finest strategy games of the 8-bit era.  Taking all the levels into account, and the ability to play as the enemy with a code, you're easily looking at 30+ hours of gameplay.

But let's talk about the gameplay.  It's simple and brilliant, focusing on careful troop deployment and taking advantage of terrain, without getting bogged down in the micromanaging of resources that afflicts many of the more "advanced" strategy games today. If you're fortunate enough to have a TG-16/PC Engine, this is a must-own title (known as Nectaris in Japan).