We propose a simple and reproducible method for fabricating large area metal films with inter-connected nanostructures using a combination of colloidal lithography, metal deposition and a template stripping technique. The method is generic in the sense that it is possible to produce a variety of nanostructures and the corresponding inverted nanostructure in a metal film. Within plasmonics this feature opens up obvious possibilities to design and fabricate substrates with specific and tailored optical properties applicable in optical sensing platforms. In this report the method has been employed to fabricate metal films with inter-connected nanostructures consisting of either partial spherical shells or the inverted structures: spherical cavities. The substrates are characterized by optical reflectance and transmittance spectroscopy. We demonstrate, in the case of partial spherical shells, the possibility of tuning the localized surface plasmon resonances which is a desirable option when applying the substrates in e.g. surface-enhanced Raman spectroscopy. Additionally, it is demonstrated that the nanocavities exhibit distinct sensitivity with respect to the refractive index of the cavity thus verifying the potential applicability of the imprinted cavities as highly localized refractive index sensors in e.g. bioscience.