I've got a few ideas, some of which have already been picked up but others haven't so...
--As it has been pointed out, the bald man's accomplice/'heartless' resembles the dark or transformed form of Riku. Also, the 'heartless' appears to be wearing a helmet almost identical to those worn by the knights, but without the decoration. This could show some connection between him/it and the knights (I've built on this idea below).
--The battlefield appears to be the crossroads. My interpretation of the crossroads has always been that the paths symbolise the four roads; the road to light, the road to dawn, the road to twilight and the road to dusk/darkness, while the crossroad itself symbolises the opportunity to change from one road to the other (think about Riku's appearences at the crossroads). If we build on the idea that the knights are good, we could presume that they embody the roads to light, dawn and twilight. If you consider the fact that the 'heartless' wears a helmet like the knights, its plausable he'd/it would embody the road to darkness. You could also interpret the fact that the 'heartless' wears a helmet is because he/it had a connection with the knights, perhaps even been one of them, but defected to join the bald man.
Nothing groundbreaking but I haven't seen anyone make the connection between the crossroads and the four 'paths' that come up during the games, and the possible implications this might have.
