After playing "Dungeon Defenders" for a bit, you'll unlock pets like this fairy hovering just behind the apprentice.

Even though the game industry is shifting toward lower-cost downloads, it still holds true that if you want a deep gaming experience that can last 15 hours or more, you’re probably going to end up paying at least $40 for a recent title.

Even the best Xbox Live Arcade or PlayStation Network games are cheap dalliances, designed to give you five to 10 hours of playtime for your $10 to $15. That’s not the case with “Dungeon Defenders,” which sells for $15 on the PC, Xbox 360 and PlayStation 3.

I’ve sunk about 15 hours into Trendy Entertainment’s mashup of the role-playing and tower defense genres, and I’ve yet to unlock a single achievement on the Xbox 360. The cute, colorful game in which child warriors protect a magical crystal from waves of ogres, wyverns and goblins features an absurd amount of loot and customization. The number of maps and gameplay customization available to players is astounding for a downloadable title.

The “Dungeon Defenders” story is simple and is relayed in a short movie when you launch the game. A bunch of children, assigned castle-cleaning duty by their adventure-hungry parents, accidentally unleash sealed-away evil in the form of a variety of classic fantasy bad guys. The kids must protect the castle’s valuable Eternia crystals from the advancing hordes.

Trendy Entertainment's "Dungeon Defenders" supports up to four players locally or online.

Like Double Fine Productions’ “Trenched/Iron Brigade,” “Dungeon Defenders” features a blend of character-oriented action and tower defense. The game’s young heroes can deal plenty of damage on their own, but the key to success is the construction and maintenance of defenses at key control points on the map.

Four character classes each have their own styles of attack and defenses to construct. The squire specializes in up-close combat and can build a variety of medieval-looking constructs, from ballistae to catapults that send giant cannon balls to bowl over enemies.

The apprentice slings magic from a distance and builds towers that shoot fireballs and electricity. The huntress is armed with ranged weapons and specializes in devastating traps. The monk mixes close-up and distant attacks while buffing his allies or weakening his foes with a number of different “auras.”

“Dungeon Defenders” features about a dozen different maps, with four difficulty settings for each. Once you’ve mastered those, you can enable a “challenge” mode that switches up the gameplay in a number of ways. You may have to defend a crystal that teleports around the map, for example.

There’s also a “survival” mode in which you must fend off increasingly large numbers of bad guys, and a “pure strategy” game type in which players erect defenses but can’t attack.

You'll earn a lot of loot in "Dungeon Defenders." Thankfully, the game allows you to lock items you want to keep and sell off everything else en masse.

As you clear stages, you’ll stockpile weapons and armor that can be sold off or upgraded to fit particular play styles. You’ll also unlock “pets,” little creatures that hover near your character, attack the enemy and provide a number of beneficial effects.

Played solo, “Dungeon Defenders” can feel like a bit of a grind, which means it’s best in small doses. Leveling up your character, weapons, armor and pets requires a good investment of time, and the game’s learning curve can be a bit steep.

When playing with friends, though, it’s easier amid the chatter to lose sight of the experience/level grind and focus on coordinating strategy, swapping gear and defeating the bad guys. (Build time between waves of enemies can be set on “infinite,” allowing plenty of time for tactical discussion.)

In fact, playing with friends is so much fun, it made me wish that Xbox Live Arcade allowed users to buy multiple copies of the game to give away the way Valve’s Steam service does on PC. Maybe someday.

For this review, a friend and I played digital copies of the game provided by the publisher, Reverb, for review on the Xbox 360. We goaded more friends into buying the game to play with us after we liked it.

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