Quick Look: Fieldrunners is an iPhone/iPad app that's been ported over to the Roku. It appears to be an arcade-style game, but it's more strategy than arcade. I've only had a chance to play a few levels (this is just a "quick look", after all), but the basic strategy can be summed up as follows.
The enemy sends troops out from one side of the screen and your job is to keep them from making it across the playing field. Every enemy killed gains you money, every enemy to make it off the right side of the screen reduces your life points. You use money to buy weapons towers that you place somewhere in the field. The weapons, such as a Gatling tower, goo tower, and Tesla tower, automatically fire at the enemy as they advance across the field. You can upgrade the towers and add new ones as long as you have the money to do so. You don't have to fire the weapons, but you do have to select the best location for them to maximize your killing power.
Does this sound like your type of game? Don't bother trying to figure it out from my description, you can try it yourself for free here, but you'll need the Google Chrome browser to do so.
Developer's Channel Description: The award winning tower defense game is now available on Roku with HD graphics! Join in on all the fun now! In Fieldrunners, you control the battlefield - Use an exciting arsenal of weapons to stop the Fieldrunners from taking over, so that you too can become the ultimate tower defense...
CHANNEL STORE CATEGORY: Games
DEVELOPER: SUBATOMIC STUDIOS
FEES: $2.99 one-time purchase. Purchase on-screen from within the Roku Channel Store.