FWIW My bucolic tip is that experience shows the cast iron plugs can break, so PB-Blaster is good insurance against drilling and extracting. Two types of square plugs exist, internal and external. Either way the best tool to use is a socket extension backwards that closest matches the size and a adjustable jaw (crescent)wrench on the extension flat tangs or use a flex handle with the extension in the drive hole. If you have an internal hole that has been destroyed, use one of the pipe nipple removers Sears sells that have a standard nut size drive. Outside rounded off, ditto with a rounded nut remover.
Avoid impact tools if you can as there is no feeling that things are going to go snap.