What are the strengths of the Phalanx?

well they are weakest when facing an enemy fielding lighter and more flexible troops

If a Phalanx was attacked from the front, it presented a virtually impenetrable hedge of spears. The close-packed mass of men also gave a Phalanx great physical momentum in an advantage or change, allowing it to physically force opponents back. Lastly, all ranks other than the front few held their spears or sarissas upright, helping block any missile fire.

Of course, these could be countered by the fact that a Phalanx was extremely vulnerable to a flanking or rear attack by a more flexible cavalry or infantry formation.