When I lived in Spain, my apartment was heated with a kerosene space heater. It did a nice job warming my small apartment. It was typical for Spain, uninsulated with single pane glass and it wasn’t well sealed so there was no issue with CO accumulation from the heater.

If your apartment is insulated with double-pane windows you have an advantage. Insulation can go a long way to making an unheated dwelling more livable. If you decide to get something like a kerosene space heater, get a battery operated CO detector at the same time.