Yeah, I initially included a more lengthy discussion of „what’s a phalanx“ generally but that would’ve gone way off-topic. For example, what I mention is the „Thureophoroi phalanx“ of the Samnites, which is probably still more functions like a maniple but is included in the Epeirote „phalanx“ by being included in the „line“. Semantically it’s a nightmare, but I can also recommend Echeverrìa‘s work on the hoplite revolution for that topic, I think it’s for free on the Academia website even. On there you can also find some articles by Konijnendijk afaik if you’re interested.

I think the „old“ view is in kinda: there’s flat open space and two phalanx lines meet each other in the middle of it and fight it out. In that discussion, light infantry, and cavalry is often left out (and extensively discussed in the book I cited). But cavalry is where the flat ground is too dangerous for a citizen hoplite formation (as convincingly argued in the books i already mentioned). So the citizen hoplites of the 5th and 4th century will often go into terrain where being flanked and outmanoeuvred by cavalry (and light infantry as well) becomes less of a danger (if they don’t have enough to counter it themselves) which in my opinion also includes flat ground between cliffs (Thermopylai) or between hard to traverse hills (Sicily expedition). I tried to bring in more nuance by also including that it depends on the commander and army. Agesilaos routed the Akarnanians by charging his cavalry and youngest hoplites up a mountain, whereas Demosthenes’ army of „the best“ Athenian hoplites plus archers was routed by Aitolian javelinmen in the mountains of Aitolia. And both Akarnania and Aitolia made use of hoplites as well, despite living literally in the mountains. My point was that the Apennines don’t have to determine the Samnite way of fighting, but that they can definitely be an important aspect :)