It has long been known that iron undergoes a phase transformation from body-centered cubic/ $\alpha$ structure to the metastable hexagonal close-packed/ $\epsilon$ phase under high pressure. However, the interplay of line and planar defects in the parent material with the transformation process is still not fully understood. We investigated the role of twins, dislocations, and Cottrell atmospheres in changing the crystalline iron structure during this phase transformation by using Monte Carlo methods and classical molecular dynamics simulations. Our results confirm that embryos of $\epsilon$ -Fe nucleate at twins under hydrostatic compression. The nucleation of the hcp phase is observed for single crystals containing an edge dislocation. We observe that the buckling of the dislocation can help to nucleate the dense phase. The crystal orientations between the initial structure $\alpha$ -Fe and $\epsilon$ -Fe in these simulations are ${\left\{110\right\}}_{bcc}\left|\right|{\left\{0001\right\}}_{hcp}$ . The presence of Cottrell atmospheres surrounding an edge dislocation in bcc iron retards the development of the hcp phase.