Catalytic and optical properties can be coupled by combining different metals into nanoscale architectures where both the shape and composition provide fine-tuning of functionality. Here, discrete, small Pt nanoparticles (diameter = 3 - 6 nm) were grown in linear arrays on Au nanoprisms, and the resulting structures are shown to retain strong localized surface plasmon resonances. Multi-dimensional electron microscopy and spectroscopy techniques (energy dispersive X-ray spectroscopy, electron tomography and electron energy-loss spectroscopy) were used to unravel their local composition, 3D morphology, growth patterns, and optical properties. The composition and tomographic analyses disclose otherwise ambiguous details of the Pt-decorated Au nanoprisms, revealing that both pseudospherical protrusions and dendritic Pt nanoparticles grow on all faces of the nanoprisms (the faceted or occasionally twisted morphologies of which are also revealed), and shed light on the alignment of Pt nanoparticles. The electron energy-loss spectroscopy investigations show that the Au nanoprisms sustain multiple localized surface plasmon resonances despite the presence of pendant Pt nanoparticles. The plasmonic fields at the surface of the nanoprisms indeed extend into the Pt nanoparticles, opening possibilities for combined optical and catalytic applications. These insights pave the way towards comprehensive nano-engineering of multi-functional bimetallic nanostructures, with potential application in plasmon-enhanced catalysis and in-situ monitoring of chemical processes via surface-enhanced spectroscopy.