Droidekas are the Heavy Battle Droids used by the Confederacy of Independent Systems, and Federacy of Machines. They were Guards that can go there, Droidekas are used to Form of a Rolling Wheel, and turn into a Robot mode.
Droidekas are First Used by Confederacy of Independent Systems, Whence they armed with Blasters, Ability to Protect as Force Field and Turning into Wheel Mode.
Droidekas were Used during the Clone Wars, in the Sieges of the Galaxy, Where Droidekas arrive on the Battle of Xenonaira and Used to Roll down and Upwards towards its Land. While the Battles are Used and Ranged to attack Clone Troopers in the Process, As they Killed to oblivion.
Following Order 66 Issued by Sally and VIKI, The "Good" Clone Troopers turned Evil and nearly destroy the Jedi Forces (Later Revived by Kota), Droidekas with all but of CIS Forces were Still Affiliated by the Dark Universal Empire, Since The Republic Reorganized into Empire.
Some CIS Forces, Including Droidekas, and its Variants will continue during the War. Hence, CIS Forces Now Joined with the Galactic Empire, and track down Rebel Troopers during the Wartime.