this thread and the youtube video had me doing a whole lot of reading and remembering.
i can remember watching a BBC documentary about the romans in england, and their light artillery, which was a really stupidly large crossbow on wheels that fired a tiny little heavy bolt, and needed a few guys to crank, aim, load and fire.
like people are saying, it wasn't that fast, to move, but it didnt have to be. the roman army was a lumbering monstrosity, and they could use their supersize crossbow whenever they picked their battles, which was whenever they possibly could.
and also like people were saying, it was a morale killer that strongly discourage charging. they found remains of celts with the iron bolts stuck right through their spinal cords, and skills with neat little holes in one side and out the other. they tested a modern reconstruction against roman armor, however, and the segmented flexible breastplate held.
aparently a big part of the terror was in how magical they seemed to the barbarians on the receiving end. they couldnt come up with their own big crossbows and things, because as the university engineering students i know of who recreate these things tell me, its really hard to make siege machinery which doesnt teari itself apart on first use and kill half the operators. and even if they captured the romans huge bows, they wouldnt penetrate the roman armor. perhaps cause internal mashing, but no piercing to speak of.
aparently archimedies the greek all round smart guy designed a complex gatling gun like spear launching machine, along with his legendary array of giant mirrors for burning ships. the romans got a lot of their good stuff off the greeks, though aparently archimedies particular automatic spearthrower was a bit complex for them. theres no excelent historical data on the ship burning mirrors, and most historians dismiss it.
giant ship burning mirrors are totally off topic from crossbows and siege bows, but still crazy awesome. even while a lot of physicists and other sciency types say it wont work, there are links and photos out there of a 1973 study where a line of 80 or so greek sailors from the navy all point a big flat mirror at a boat 50 metres away and it bursts into flames. some people at a historical society somewhere i forget did this with a ships sail and a university professors help with figuring out focal distances and stuff, and some solar power research guy used 100 bent mirrors in a field to burn off the tops of some trees he didn't like. most reliable info is Archimedes may have used a lot of hexagonal mirrors all linked together with hinges in a dish shape to set fire to roman ships a bowshot away. , even if nobody is sure its true. ill find the links for the above stuff tomorrow if people want.
so maybe when we get good sieges with siege towers and wall climbers, we can have our uber metalsmiths and mechanics make giant hinged segmented dishes of polished silver, and use them as a death ray to torch the goblin hordes and set aflame their advancing siege towers.
or, picture one flaming arrow flying forth to mark a target, and then atop your walls 100 stout dwarves with polished reflective shields swinging into position to focus a narrow beam of death upon the invaders and flame everything before them.
gosh i type a lot.