An extensive numerical study of natural convection flows arising from a horizontal cylinder centred in an air-filled, square-sectioned enclosure is presented. In the frame of the 2D assumption, a specifically-developed simulation procedure is adopted, with the aim of achieving a detailed description of the transitional dynamics. The sequence of bifurcations marking the transition of base fixed-point solutions to unsteady, chaotic flows is followed for increasing values of the Rayleigh number, and for two values of the enclosure aspect ratio A, defined as the ratio between the enclosure side length L and the minimum cylinder-to-wall gap H. A rich variety of peculiar dynamical features is observed, which appear as quite uncommon in thermofluid systems. The correspondence between system dynamics and flow and thermal patterns is also highlighted.