Urbanisation and coastal armouring are particularly evident in cities and ports, with large proportions of coastlines transformed to artificial vertical and sloping seawalls. Hard eco-engineering structures, which provide microhabitats (e.g. crevices, water retention), can mitigate heat and desiccation stress to marine organisms, enhancing biodiversity on seawalls. A polluted marina in Hong Kong with both sloping and vertical seawalls was chosen as a case study for hard eco-engineering in a degraded ecosystem with continuous influence of contaminated surface runoff and the maritime industry. Eco-engineering fixtures that provide physical refuge and water retention or high complexity via an oyster shell reef were placed on the sloping seawall with respective control boulders, and two designs of complex eco-panels were placed on the vertical seawall, with a control panel and scraped seawall control (n=4). Following 12 months of deployment, the species diversity found on most of the sloping seawall fixture types and both panel types were significantly higher than the respective controls. The significantly different community composition of the fixtures also shows that eco-engineering fixtures can improve the biological community and ecosystem functioning on the seawalls in a polluted marina, pushing the limit of eco-engineering in marinas and busy ports of urbanized coastal cities.