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This is hands-down the best 99 cents I've ever spent. Just one problem: the game is so good, it's starting to eat into my life.
Basically, it's a fixed-position TD game (as opposed to a mazing-style game) with a high degree of customization and a degree of strategy that is at odds with it's inviting graphics and easy learning curve. You have 3 major facets to gameplay.
1. Your towers (duh) They come in 4 varieties: Barracks (spawns soldiers to block the road and fight enemies), Mage Towers (does magic damage, which ignores armor), Archers (High range and DPS, my personal favorite), and Cannons (Huge damage and AOE, slow fire rate). Each of these splits into two different styles of tower after the 3rd upgrade, and most towers can be upgraded at least 8-10 times total.
2. Your Hero You have a total of 3 that come with the game and are unlocked as you progress. You can buy others with real-world money, but they're kinda pricy (I think there's a dragon that's like $10, pretty steep considering it's basically a $1 game). They have unique abilities, ranging from healing allies to summoning reinforcements, and some of these are better suited to different levels. Experimentation will help a LOT.
3. Upgrades -Towers/CD Abilities You can spend the points you earn in battle to upgrade both your hero and your towers. In the tower upgrade menu, you'll find the upgrade charts for your Rain of Fire (a devastating AoE attack that can be summoned independently of your towers/hero) and Reinforcements (another independently-operating facet of gameplay, you're able to summon 2 warriors to "hold the line" every 10 seconds or so) in addition to your towers. I'd recommend fully upgrading the Archer towers first, and then moving on to the Rain of Fire. These are the two most important pieces of gameplay early on. -Heroes You can upgrade your heroes with new abilities every time they level up. Different abilities cost different amounts of points, and points are earned when they level up as a direct result of experience gained damaging/destroying enemies. If they're just sitting in the back the entire time, not using any of their attacks, they won't gain any experience. You can also purchase additional heroes, ranging from a pirate to an alien to a dragon.