The iPad is a perfect platform for Tower Defense, as War Grimoire HD demonstrates. The large screen (for mobile platforms) and multi-touch capabilities make it easy to appreciate the lay of the land you're defending. War Grimoire HD sports a very colorful cast of both defenders and enemies, giving you incentive to pinch-zoom in on the action at any time. Another nice touch visually is that the appearance of each unit will change depending on how you've outfitted them and what level they've currently attained. There are a few sporty musical themes, but the main reason to keep the volume dialed up is to hear the distinctive "chink" of item drops. Aside from the main game mode, you'll find nice variation in a series of mini-games, unlocked as you progress through your campaign. The only drawbacks are some visual glitches or freezing that accompany more serious crashes. Whether this is happening because of display issues or core programming is impossible to know, but it puts a damper on what is otherwise a nicely polished production.

If you don't like a lot of prologue, War Grimoire HD is the game for you. Only a brief movie that shows a few of the game's main characters stands between you and the first level. After all, Tower Defense (TD) is always about you versus the other side, so is it really critical to know why we're fighting? There's only one option at first, to begin a new game and start plowing through levels. By the time you reach the end of the first series of levels, you will have unlocked a mini-game. These range in style from card-based memory games to more active arcade style shooting games. In all cases, you can use mini-games to amass points and unlock new items, which should give plenty of incentive. Plus, they're fun...

The core game is a great example of how TD blends puzzle elements with real-time strategy. During each stage, you'll play through a series of levels that introduce enemy movement and friendly troop placement. Some levels feature very linear options for troop movement, while others have troops coming from multiple directions across the screen. In all cases, your job is to stop them dead in their tracks. There are only six distinct units, but you have the ability to upgrade them and equip them with a wide array of weapons. This is where strategy really comes into play, and where you can customize War Grimoire HD to suit your personal playing style. Sure, there are better and worse ways of completing each level, but there's no right or wrong approach. As mentioned earlier, the current version of the game (as of June 2011) tends to crash at times, causing you to instantly lose progress in a level. Very frustrating, but something we've seen in other games that can generally be fixed through app upgrades.

Tower Defense as a genre tends to be hard. Mostly due to user-error, of course. Unlike action games that feature some form of unlimited ammo or attack limited only by how fast you can press buttons, TD always grants limited resources and gives you one, limited-time opportunity to destroy enemies. When the resources are tapped or the enemy has passed, you're out of luck. War Grimoire HD exploits all the classic TD devices, plus it adds a dimension of persistent upgrades and items. Generally, you see more games in this genre where many units are available for use, and may be upgraded during a level with proper resources. In the War Grimoire HD model, there aren't many units, but you can use the spoils of war to buy equipment that changes the nature of each unit. Attack-based units can become magically endowed, various units can benefit from increased speed, and special abilities like slowing enemies or penetrating armor can be granted through equipment. Even on the easiest difficulty setting, most players will find War Grimoire HD challenging. Dialing things up grants you more points and significantly more challenge, and we'd wager not many players will need anything more than the so-called "Normal" setting.

Most of the controls in War Grimoire HD are intuitive, but there are a few idiosyncrasies. The first thing we found annoying was the way each level begins. In general terms, you have about 15 seconds to place new units before the first enemy wave arrives. In theory, this might make sense, but the controls are a bit syrupy. There's no grid on the screen showing where units can be placed, so tapping once to select a unit and tapping again to place one will occasionally fail. Placing a unit and upgrading it takes a few more taps, and you'll want some units upgraded immediately. We want more responsive controls in general, and War Grimoire HD scores a B or B- on this front. When your units are plentiful and you are well-stocked for defense, it's not as noticeable, but you'll still reach a point where you just want things to move faster. You can speed up the flow of the action, which helps when you're grinding for points on easier levels, but this doesn't change the response time for taps, swipes, and other commands.

War Grimoire HD isn't always as polished as we'd like, and the occasional crashes will be a major problem unless fixed as part of a future update. That said, if you can overlook its few quirks, you'll find a well-designed strategy title that should appeal to any fan of Tower Defense. At times crushingly difficult, to the point that it verges on cheap, War Grimoire HD offers plenty of incentives to keep working toward mastery, whether you're a veteran strategy gamer or a complete novice.