Maybe I should have been more specific. "Solid" is a very general adjective and can mean very different things in different contexts. "Solid" here is an adjective modifying the word "phalanx," which is an attack formation used by ancient Greek soldiers where they stood very close together in a line so that their shields overlapped one another's. So in this case a solid phalanx means a strong unbroken line. "Solid armed guards" would be a case where "solid" modied "guards," and saying a person is "solid" means the person is heavy and muscular.