This study investigates the interplay of terrain, blockage, and wake effects using Reynolds-Averaged Navier-Stokes (RANS) simulations of 35 different combinations of terrain, wind farm layout, and atmospheric conditions. The terrain includes two idealized solitary ridgelines, an idealized valley, and flat ground. The wind farms comprise one or two rows of closely spaced turbines parallel to the terrain feature. We simulate these idealized wind farms in conventionally neutral boundary layers of different heights. The set of simulations also includes an existing onshore wind farm located along a ridgeline and run with stable and unstable surface conditions. The horizontal variation of the ground elevation (i.e. terrain) has a large influence on wake and blockage effects in this study. In addition, the predicted wind farm efficiency and turbine efficiency (power coefficient) vary significantly depending upon the terrain in the simulation and the position of the wind farm relative to the terrain. For single-row wind farms the predicted effect of terrain on wind farm efficiency can exceed 4% – for the simulated conditions. The separate but correlated effect of terrain on individual turbine efficiency is of a similar magnitude. Analysis of the results indicates that there are multiple physical drivers behind the efficiency trends, including streamwise pressure gradients and inviscid effects related to buoyancy. Energy prediction methods that do not account for these drivers have an elevated risk of producing large errors – at least at wind farms similar to those evaluated in the study.