Flags and horns mostly.
Designated soldiers would carry flags or banners to indicate certain information.
Like “march after the guy with the flag”
More timing sensitive actions could be signalled with horns too.
Like “release the barrage of arrows from the hidden ballista”
And of course the leadership structure of the army was important too.
If you have good sergeants leading smaller groups, you almost dont need a general waving his flags
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