This is a setting with a creepy motif. The buildings are gothic in design (often clearly modeled on The Cabinet of Dr. Caligari), there's always a heavy fog in the air that obscures your vision, and the full moon always lights the cloud-draped sky... even at 2:30 in the afternoon. Halloweentown is usually inhabited by the usual assortment of horror tropecreatures. The main color schemes are black, gray, orange,red, and black and slimy green, and black... sometimes in combination.
Despite the somber colors and the Horror Trope decor, this setting is not always played solemnly. Imagine the Perky Goth or Nightmare Fetishist character as a setting, and you've got this place. It's less Eastern European and more trick or treat. Halloweentown is usually not that scary, at least not intentionally so. It's intended to be somewhat playful and fun. A good way to tell if a series is set in a place like this is if there are jack-o-lanterns and it's not Halloween, although it's not a prerequisite.