Breakdown of the ice rule in artificial spin-ice nanostructures results in magnetic monopole defects with zero magnetic moment. Such defects exist during the magnetic switching process in some nanostructures and yet are absent in other apparently similar arrays having the same geometry and made from the same material components. One explanation proposed for this discrepancy is that it is due to the variation of disorder across samples, with monopole defect formation occuring only in highly disordered samples. Although disorder can indeed play a role in the determination of monopole density, in this paper we show, by experiment and simulation, that in samples of similar, low disorder, the factor controlling the nature of magnetic switching is whether the domain walls are in the transverse wall regime or in the vortex wall regime. This work illustrates that monopole formation can be controlled by intrinsic micro-magnetic behaviour as well as by extrinsic quenched disorder.