The core of this thesis explores the optical response and tuneability of plasmonic cavities. At its heart stands the nanoparticle on mirror construct (NPoM) - a robust plasmonic cavity formed via self-assembly. It consists of a nanoparticle placed on top of an under- lying metal substrate separated by a nanometre thin spacer material. Plasmonic cavities have the ability to confine light beyond the diffraction limit via collective charge oscilla- tions at dielectric-metal interface. This thesis explores different ways to boost, localize and exploit nanometric light-matter interactions in the NPoM configuration. Due to their large field enhancement reaching values of the order of 10^4 and mode volumes as small as 40 nm^3 they form a versatile sensing platform. A custom-built polarization dark-field microscope is used to probe the morphology of the NPoM cavities on sub-nanometre length scales. Comparing the ex- perimental results to boundary element method (BEM) simulations the NPoM system is fully characterized and the influence of the cavity morphology and faceting of the NPoM is investigated. A spectral splitting in the polarization signature of narrow gap systems (<1 nm) is directly related to the asymmetry and orientation of the plasmonic cavity, revealing its orientation and asymmetry which is otherwise inaccessible for conventional imaging techniques. Furthermore, laser irradiation is employed to induce atomistic re-sculpturing processes of the cavity morphology, actively tuning the plasmonic modes. Two regimes of growth are identified: facet growth using hard spacer materials and conductive bridging using soft or porous spacer materials. The latter is used to monitor the controlled formation of conductive bridges in memristive devices. Finally, incorporating mono- and multi-layers of 2D materials into the plasmonic cavities, light-matter interaction is studied on the nanoscale. Stable room-temperature excitons of WSe_2 allow the strong-coupling regime to be reached in ambient conditions. Exploiting the large field enhancement of the NPoM cavity and tuning its plasmon modes over the exciton resonance reveals the typi- cal anti-crossing signature of a strongly coupled system. This combination of ultra-small mode volumes of plasmonic cavities and large coupling strength of 2D materials offers a promising route to realistic exciton devices and allows a glimpse into the quantum world at room temperature.