The Destroyer Droid is an Trooper that is unique to the Trade Federation, and is available at the Fortress in Tech Level 3. It is more effective against Troopers than the Strike Mech.
In Tech Level 4, it can be upgraded to the Hvy Destroyer Droid, which has more attack, armor, pierce armor, and range, as well as shielding.
"Exclusively manufactured for the Trade Federation, the droideka — also known as the destroyer droid — has an innovative design that lets it travel fairly quickly (by curling up and rolling) over all terrain. It may willingly unfurl and mount a powerful assault. They are even better than Strike Mechs against enemy infantry. Heavy Destroyer Droids gain personal shielding."
—Manual description
Gallery
Hvy Destroyer Droid icon.
In-game Destroyer Droid.