True to the game's minimalist approach, all enemies are abstract shapes -- circles, triangles and so on. Similar to Asteroid, the large circles break up into smaller ones once destroyed. Moreover, enemies with different colors react differently to the walls. Although the first 2-3 levels are easy to the point of tedious, the pace picks up quickly from there, and soon you will be struggling to deal with all sorts of bouncing and self-dividing enemies.

DUO adds a number of nice features that, while not revolutionary, are not common in SHMUPs. At the end of each level, you are awarded with points for a number of areas including accuracy, as well as the number of times you managed to avoid being hit or catch power-ups. In addition to points, you are also rewarded with time bonuses, free weapons upgrades, and shield recharges. Another nice feature is the ability to save every 5th level - a welcome change from typical shooters that force you to restart from the beginning.

There are 3 game modes in DUO: Challenge (the default), Time Attack, and Survivor. If you manage to finish the Challenge mode, the other two modes add significant replay value, and make the game well deserving of a permanent spot on your hard drive. Last but not least, you can turn off special effects if you think they distract you from the game, and load up your own music to listen to.

DUO is a very polished freeware game in every respect. I couldn't find fault with it, other than a few minor, and very rare, bugs that crash the game once in a blue moon. Highly recommended to SHMUP fans.