Three-dimensional direct numerical simulations are used to characterize turbulent buoy- ant convection in a box-shaped Rayleigh Bénard cavity with a rough bottom plate made of a series of square based blocks separated by valleys. The cavity is filled with water, and the Rayleigh number varies over five decades up to 10 10 . As mentioned in the literature, three successive heat transfer regimes are identified: from inactive roughness (I) to a regime (III) where the heat transfer increase is larger than the one expected from the only roughness induced surface increase. The heat transfers of the transitional regime II are particularly intense. After validation against experimental and numerical data from literature, we highlight the role of the inner fluid trapped within valleys. It is shown that the heat transfer through the fluid interface between the cavity bulk and the inner fluid is strongly related to the overall heat transfer at the rough plate, with an exponent of the heat transfer scaling law close to 1/2 in the regime II. We found that this regime is active when the thermal boundary layer is thinner than the roughness height and, at the same time, the kinetic boundary layer is larger. As compared to regimes I and III, regime II is characterized by larger temperature fluctuations, especially near the rough plate, and a larger friction coefficient. A fluctuating rough layer overlaying both blocks and valleys appears in the regime III, in addition to the classical boundary layers formed along the plate topography.