Koopa Soldier

Koopa Soldiers are Koopas specially trained to gaurd "Bowser's Fortress". They are trained at "Bowser's Boot Camp", where they go for 3 years until they are ready gaurd it.

Other places were Koopa Soldiers are found
Since regular Koopa Troopa's fight against mario in the plains, and Dry Bones guard Bowser's castle, there really is nowhere for them to guard except Bowser's Fortress, except for Bowser's other bases, such as his Navy base, Air Force base (Yes, Boswer realy wants Mario killed that bad), etc.