  Assembled arrays of individual atoms with Rydberg-mediated interactions
provide a powerful platform for the simulation of many-body spin Hamiltonians
as well as the implementation of universal gate-based quantum information
processing. We demonstrate the first realization of Rydberg excitations and
controlled interactions in microlens-generated multisite trap arrays of
reconfigurable geometry. We utilize atom-by-atom assembly for the deterministic
preparation of pre-defined 2D structures of rubidium Rydberg atoms with exactly
known mutual separations and selectable interaction strength. By adapting the
geometry and the addressed Rydberg state, a parameter regime spanning from weak
interactions to strong coupling can be accessed. We characterize the
simultaneous coherent excitation of non-interacting atom clusters for the state
$\mathrm{57D_{5/2}}$ and analyze the experimental parameters and limitations.
For configurations optimized for Rydberg blockade utilizing the state
$\mathrm{87D_{5/2}}$, we observe collectively enhanced Rabi oscillations.
