[@Aisling] Absolutely no space for that... I've got my desk, my bookcase, my bed, my commode (with a TV on it), a full wall shelf, a CD player, and two small boxes with decoration in here. So nothing unnecessary, but still... I've just asked some Buddhist friends. They said that the windowsill is okay if there is no other solution.

I'm not so sure about it. I'm relatively new to Buddhism... I only know that I need a plant and a candle.