What's a gamer to do in the summer, when so few high-profile retail titles are released? I look for new challenges to download - and most of those are going onto my Xbox 360. It remains the essential machine for gamers who don't want to leave home.

Microsoft's Xbox Live has just launched a Games on Demand service, which lets you download older hits like “BioShock” and “Burnout Paradise” directly to your hard drive. It now has an Indie Games section filled with low-priced experiments in innovative play. And the prime-time game show “1 vs. 100” has become a smash, drawing as many as 445,000 players in one night.

But Xbox Live Arcade is still the core of Microsoft's online strategy, as demonstrated by its second annual “Summer of Arcade” promotion. This year's releases have been a mixed bag: I enjoyed the goofy platformer “'Splosion Man” and the motorcycle racer “Trials HD,” but could have done without the high-def remakes of “Marvel vs. Capcom 2” and “Teenage Mutant Ninja Turtles: Turtles in Time.”

Microsoft saved the best for last, though. “Shadow Complex,” the final entry in “Summer of Arcade 2009,” is one of the finest games of the year.

This $15 adventure harkens back to beloved 1990s games like “Super Metroid” and “Castlevania: Symphony of the Night.” They're what we called “2-D side-scrollers”: You could move up, down, left or right, with the screen scrolling to follow your character. (“Shadow Complex” is really “2.5-D”: The settings have three dimensions, but you can only move in two.)

Your character is a hiker named Jason Fleming, who, with his girlfriend, stumbles across a massive paramilitary complex in the mountains. Your sweetie is taken hostage, so you have to storm the facility - armed only with a flashlight.

Of course, you'll quickly discover more lethal weapons, starting with a pistol and working your way up to a missile launcher. You'll also find armor, grenades, explosive foam and other devices that will help you survive the waves of troops and robots that come after you.

Combat isn't the main attraction, though. “Shadow Complex” is essentially a giant maze filled with clever puzzles and hidden prizes. Some parts of the maze can't be solved until you find certain devices, like a rappelling hook or hyperspeed boots. You can race through the game, but it's more rewarding to take your time and explore all the nooks and crannies.

And once you're finished, you can tackle a series of “challenge rooms” where you can stress-test all your cool new gear, challenging your friends to get the highest scores or fastest times.

The story here isn't much, and the shooting isn't as crisp as you'd get in a 3-D game like “Gears of War” (from the same studio). But “Shadow Complex” delivers more fun than most games that cost four times as much. Three-and-a-half stars out of four.