war cry
閧 centers on a loud, collective shout raised in battle: a war cry.
閧 combines 門 (gate) with 共 (together), suggesting a unified shout raised at a gate or in battle. The exact historical development is uncertain, but the component combination fits the meaning of a collective war cry.
Inside the gate 門, people gather together 共 and let out a loud war cry.
For コウ, imagine a crowd at the gate shouting a war cry that sounds like a deep 'KOH!' — the invented shout 'Koh' cues コウ.