Tower Defense games are beginning to feel very ordinary, so it's nice to find something that pushes the envelope. The big difference in appearance is the vertical bias of the game, compared to the top-down perspective that most have presented up to this point. The logic has been that you needed a broad, sweeping view of the action to know when and where to add or upgrade units. This is still true, so there are times here when you find yourself scrambling to scroll up or down to address action happening in remote parts of the battlefield. Defender Chronicles HD does a nice job of keeping the fields of battle compact enough so you can quickly adjust in these moments, but there aren't enough indicators, visual or otherwise. The waves of enemies can appear in unexpected places on-screen, which makes things fun once you know the layout and can devise a strategy.

The battle animations are simple but effective. Defender Chronicles HD opts out of using fancy lighting and smoothing, in favor of sprite-based graphics that lend a retro feel to the proceedings. Characters have catch phrases that may grate a bit on you, but these aren't overused and often provide some necessary feedback when you take an action like adding or upgrading a unit. Around the game, you'll find some impressive voice acting and interludes that help create a real story behind the action. Plus, if you don't like the included music, you're welcome to change the settings to play your iPod music throughout the game.

We said "pushes the envelope" earlier, rather than "breaks the mold," because Defender Chronicles HD is still following the same conventions as most any Tower Defense game, when you really break it down to the essentials. You play a hero that will bring order to the kingdom, leading the charge against an orcish army. Each level is a set piece that pits you against wave after wave of enemy soldiers, intent on storming your hero's fortress. The idea of the hero is used here a bit like we saw in Blizzard's Warcraft series, where you possess a super-unit capable of influencing the flow of battle. You can level up this hero throughout the game, even equipping items and abilities as you would in a role-playing game, versus the units that are only available during battle. Upgrades applied to units are lost after battle, since they are purchased with the credits gained from fallen enemies. The choice of hero can change how you approach battles, since each type lends itself better to some units than others, and unit selection is critical to winning.

Game choices include several modes, from the overarching Quest to Custom Game Mode, that includes variations on each level such as Freestyle, Extended, Classic, and Classic Extended. The replay value immediately goes through the roof, especially when you consider that the first two listed allow you to bring your hero and items over from the main Quest. Questing for points gives you the opportunity to purchase special items that can enhance your hero, which in turn will enhance some of your units. We thought it was a poor decision to make the descriptions of all the units purchased items, rather than including this with the standard documentation. Knowing nothing about the units you battle with goes against the grain of any Tower Defense game we've ever played, because it just leaves players in the dark. It's important to know which units have some advantage over others, and understand the upgrade paths for each, in order to plan strategy. Other than this small misfire, Defender Chronicles HD has all the right elements to remain on your virtual shelf for a long time.

There's not a lot of support for players new to the genre, and relatively little acknowledgement of the learning curve for those accustomed to top-down Tower Defense games. Not knowing the unique qualities of each unit is a hindrance, even though there are some basic explanations by way of a tutorial, during the first level of the game. Knowing that the Archer types are good at picking off targets from afar and that the Warrior types are good at slowing down enemies and attacking from close range is established, but the more pressing questions are why you would place one unit in a certain location on the field versus another, and which enemy units are weak against which of your units. Unlocking the "secret" guide to the game's character types really exposes information that is needed immediately upon starting to play, or really before you play.

The difficulty levels that can be selected by level are punishing! Three tiers are available immediately, with a fourth that can be unlocked. In the first few stages, Novice and Casual difficulty seem approachable, with Master as the stretch. After a few victories, even Novice feels challenging. This comes home in the speed of the game and the volume of enemies coming at you. Like most Tower Defense games, the objective is to halt enemies, letting as few through to the end of the level as possible. For each enemy that escapes, you lose points. When the points are all gone, the level is lost... Where Defender Chronicles HD begins to throw a considerable challenge at you is in the variety of enemy types that require a careful balance of defending units.

Playing Tower Defense on a touch-screen sounds like it would be a dream come true, but the realities are a bit different. We've become accustomed to scrolling around screens, and clicking to place units. Tapping to place units isn't that much different, but the scrolling mechanic and the placement can be tricky. Scrolling never seemed to happen fast enough, and we wanted an option to quick-scroll to places where the action was happening. Something along the lines of a shortcut key would have helped here... The placement issues may just relate to our fat fingers, but we noticed that it was hard in some cases to select the right unit for upgrades, once the field was crowded.

A single tap shows the effective range of a unit, along with options at the bottom of the screen for upgrading and selling a unit. This back-and-forth between the action and the menus at the perimeter of the screen was a poor design decision. More of a contextual menu would have been preferable, to prevent the constant see-saw between one side of the screen and another. Having to do this back-and-forth for simple things like upgrades to units seemed especially unnecessary. Most other elements of Defender Chronicles HD were intuitive, including some nice features in the world map for browsing unlocked areas where you can modify your hero, switch to a different hero, or look into upgrades.

If you're a bit tired of playing derivative Tower Defense games, you owe Defender Chronicles HD a look; it's a different spin on the genre, and a nicely polished adaptation for iPad.