From a historical stand point they should be used, but the only thing that worries me about them are the rules. Unless you guys already have some base rules for PM warfare, I think their use would be a huge complicator in battles. And also from a historical perspective, they were very expensive back then, so a cannon to klicky ratio would be an interesting feature. I think the same should apply to catapults and balistas.
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I have made a set of rules for pre-warfare fighting.
Here they are:
http://gardenwargaming.playclicks.com/forum/index.php?topic=1370.0The newest most updated version of the rules are below,
You just need to scroll down a bit.

And I didn't say we could add Romans, maybe the Egyptians,
but only if they fit in with your country.
I am saying the Egyptian armor of the time of the Romans, (like the armor in Playmobil)
Would have still been used in Medieval times.
Roman armor would not.

I also never said to put a limit on the number of Klickies,
I only said that there should be a ratio to how many cannons per how many Klickies.
I dont believe there should be a ratio to how many klickies
can control how many catapults, or ballistas.
As these did not require as much skill to make as a cannon,
and were much less expensive.
I believe that to have a catapult of ballista,
you only need as many people as it takes to operate it.
(Which according to the rules I made (should anyone use them) is 4 people.

)
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Put cheng's samurai in there, too.
Speaking of Cheng, I would be ecstatic if he joined!

Just cus it is Medieval doesn't mean it has to be Medieval
Europe.