BackgroundPublic housing estate is a key determinant of community health risk in American/European cities. However, how forms/characteristics of compact/hilly public housing's neighbourhoods affect dementia among Asian seniors was underestimated. DesignThis was a cross-sectional study. MethodsA total of 2,077 seniors living in Hong Kong's public housing estates were included. Dementia was measured by a Cantonese version of Montreal – Cognitive Assessment. Built environment was measured based on three dimensions (greenery, walkability, accessibility), including 11 metrics. Circular buffers (without walking paths) and service areas (considering walking paths) with two-dimensional/three-dimensional (terrain) adjustment were applied to quantify forms/characteristics of neighbourhoods. Two spatial buffers were applied: immediate distance (200 m) and walkable distance (500 m). Exposure-by-exposure regressions were applied to evaluate the associations between form/characteristics of neighbourhood and dementia. ResultsForms/characteristics without considering walking paths may overestimate health benefits from built environment. For circular buffers, higher percentage of building coverage, higher land use mix and more community/transportation/leisure facilities were negatively associated with dementia. All measures of greenery were positively associated with dementia. For service areas, measures of walkability and accessibility became insignificant except more community facilities at the immediate distance. Furthermore, terrain effect was insignificant when it was compared with the impacts of walking paths. ConclusionDementia among seniors in hilly public housing estates was negatively associated with neighbourhood's walkability and accessibility and was influenced by walking paths. For healthy ageing, improved forms/characteristics of public housing neighbourhoods should include more accessible spaces and community facilities along walking paths for physical activities and basic daily needs.