Droideka's aboard a Droid Control Ship

Originally built by the Colicoids, destroyer droids – generally known as droidekas -- were among the most formidable Separatist battle droids, feared for their deadly combination of speed, firepower and defensive capabilities. Even Jedi gave these killing machines a wide berth. 

Droidekas during The Clone Wars

Fortunately for the Republic, droidekas were far more expensive than other Separatist droids, and so encountered much more rarely on the battlefield. Separatist commanders used them in a range of roles, sending them into battle as heavy infantry and reinforcements and employing them as bodyguards and even executioners.

During the Clone Wars, both Jedi and clones learned techniques for thwarting destroyers, figuring out how to penetrate their shields and use their own capabilities against them.