[colorado tanker]

Yes, of course, a phalanx is quite vulnerable on its flanks. The advantage of the position at Thermopylae is that the narrow pass gave the Greeks protection for their flanks - until the Persians got behind them.

[USMMA_83]

That's exactly right. Even the comic-book movie aludes to the fact that the Spartans wanted to confront the Persians at the narrow pass exactly for that reason. In the open field, the Spartans were toast.