This is Sparta! If you have a physique that you're proud of (or even if you don't), Halloween is a great time to show off those abs with a Spartan costume. Like many of the legendary warriors of yore, finding the right elements to dress yourself won't be too hard because you won't have to wear much.
Spartan is often used as a synonym for "minimalistic," and the reason for this is obvious if you've ever seen The 300. A briefs-like brown loin cloth should suffice as far as bottoms go, and there's no need to wear a shirt up top. It's questionable whether actual Spartans fought semi-nude like this, but this is what most people these days envision when they think of the majestic Greek warrior.
If you feel a little too self-conscious about running around in what is essentially underwear, then consider wearing fustanella over it, a pleated skirt-looking thing that Greek soldiers often wore.
According to oral tradition, Spartans wore red capes because it would make their injuries less apparent, as the blood would blend into their clothes. Whether or not this is true, it has become an important part of the Spartan image, so shop for a red cape online or get some blood-red fabric from the fabric store and make your own.
What is a warrior without his sword, shield, and helmet? You can easily find Spartan or just generic Greek-style weapons and equipment in party stores. If you want to get fancier, you can also buy a replica sword and shield, but of course this will be pricier. If you want to show some skin and rush into battle this year, try a Spartan Halloween costume!