I'm shocked that nobody has mentioned one of the most devestating weapons of all time:
The Longbow.
Many people overlook this weapons as a simple bow. This is not the case.
The English Longbow has a massive, successful history, which I advise you all to read about if you're interested.
It is believed to have first been used by the Welsh in the 1100's during the invasion of Wales. The Welsh bowman exacted a heavy toll on the invaders, and were soon incorporated into the army. Men needed special training to be able to draw the string back to its full length, and often peasants were taken in and trained. Depsite being of low birth, the upper class were under orders to treat them well due to the deadly effectiveness of the weapons they used.
A fully drawn arrow could penetrate armour, and bowmen were supremely accurate (think Robin Hood)
The draw weight for one of these was somewhere between 90lbs and 150lbs - few sources agree on this. That's between 6 and 10 stone - just being held in your fingers!
Isaac Asimov once referred to it in an essay as the most successful secret weapon of all time - despite the fact it wasn't a secret.
WIthout going into details, Longbowmen basically turned the entire outcome of skirmishes and battles between the English and French during the hundred years war, with cases such as 100 longbowmen fending off an attack of 400 infantry and cavalry - with less than ten casualties.
Apparently the French once managed to create a faction of Longbowmen, but they were not well trained and treated as inferior. When they were used in battle, the mounted knights, believing them to be a waste of time, simply rode out over the formation, trampling and killing the majority. Because of this, they lost the battle, as the riders were picked off by the elite English Longbowmen.
They are supremely vague examples, but they show the effectiveness of the longbow. It wasn't until cannon was used in battle that the longbow became inferior, as the squads could be easily decimated.