This is a slightly more 2D version of popular 1D strategy games like Endless War, or Epic War, or Warlords: Call to Arms. You place your units on the battlefield and they walk straight to the left (a little odd, most of these games are left-to-right), firing at anything they can hit an stopping if they run smack into an enemy. The variety of units, firing patterns, costs, and damages provides some decent strategy, and its well-animated so as the battle gets intense its fun to watch.
The games not perfect though. The interface in general is a bit clunky. Its very difficult to rearrange your selected units (requiring removing all and then re-adding in the order you want), theres no indication of how many levels there are (only after reading the iTunes page am I told 25 normal + 25 hard). The instructions are minimal and took some experimentation to understand what I was doing, and "upgrades" are a little odd - they cost the same for all units, can only be used occasionally, and only last for the one battle.
Still, as a currently free game its a no-brainer, and would well be worth $1. Fans of the genre should pick up for even a couple bucks if it increases in price, or if they fix some of the UI issues.