Photovoltaic devices based on inorganic perovskites, such as CsPbI₃, are of great interest for applications, either as a single-junction or in Si/perovskite tandem devices due to their favorable bandgap. Such applications often require the deposition of the perovskite active layer on patterned indium tin oxide (ITO) layers. Yet, in many instances, the deposition of CsPbI₃ on structured ITO leads to the almost instantaneous degradation of the perovskite layer during film formation. In this work, we demonstrate how the microstructural and topographical features of patterned ITO substrates influence the degradation of CsPbI₃ into its nonperovskite δ-phase. By comparing two methods for patterning ITO, i.e., laser-patterning and chemical etching, we demonstrate that perovskite degradation consistently initiates at laser-formed terminations. We utilize scanning electron microscopy, electron backscatter diffraction, and confocal microscopy to prove that even nanoscale surface steps and microcrater edges, approximately 50 nm in height, are sufficient to trigger localized δ-phase formation. These regions exhibit distinct thermal and structural behavior, including recrystallization and grain coarsening. Our study provides a mechanistic understanding of how substrate morphology drives phase instability during film growth, paving the way for substrate engineering strategies to suppress phase instabilities that occur during the fabrication of inorganic perovskite-based optoelectronic devices.