Your boy is really cute, and I love your castle!
A heater really is necessary, their tank temp needs to be between 76 and 80, any lower and their immune systems suffer and they just won't live as long. (There are always exceptions to every rule...but your betta will be happier, more active, and overall healthier with properly heated water.) A heater also makes sure that the water temp doesn't fluctuate too much. Even in a room with a heater in the winter or no air in the summer can have pretty dramatic temp changes through a 24 hour period, and this is the worst for bettas. Constant temp is foremost important...jumps up and down will do more damage than even a tank that is constantly cold.
An adjustable heater is best, just a small one like a 25w, but if you just can't do that, then a preset heater is another option and usually cheaper. :)
I will admit that I am normally a 5+ gal person, but I'm not about to berate anyone over it. :) This is what you've got, and as long as it's kept clean, your boy can be very happy here. :) If you ever decide you want to cycle, a sponge filter would be the easiest to do. They're small, they don't have a current, and they're easy to cycle with a small tank like this one. :) It's not necessary, but it does cut down on your water changes, which is kind of nice. :)
I'd say the only thing you're really missing is a heater, otherwise he's looking good and happy in his new home! :)
Last edited by Blue Fish; 09-21-2013 at 11:52 PM.