Please explain how exactly to man the balista towers?

[Handel]

The nanual says "put troops close to the towers" and my archers and melee units were on the walls, but nevertheless I got the message "Our battle machines are idle, sir! We need to man them".

[Matty]

You have to be about 20 metres away - you can tell when you're close enough because the flag pops up on the top of the tower.

[Oaty]

I had that happen to but forgot what the trigger was. Was it a spy that opened the gates? As long as a tower has a flag above it your good to go.

[Vlad Tzepes]

Perhaps the message you heard was about something completely different - and it's been referred in the bug list (AFAIK).

I heard it first time ("Our battle machines are idle, sir! We need to man them") when I killed the AI units manning a ram - and it made me rofl, yeah, sure, capture that abandoned Mongol ram right now and trample those Heavy Horse Archers, that'll teach them!

For the towers - any towers, but not cannon ones in citadels, you won't get them, that's also referred in the bug list - just place unit/units on walls close to the towers, you'll see your flag happily blobbing over those buildings. And they'll fire at enemy in range (friendly fire included).

[Quillan]

And for a really humorous aside, if the unit causing the tower to be manned is currently affected by rotting cows tossed from a trebuchet, the flag on top of the tower will also flash green.