Historical accounts on the use of siege engines

Has any one come across any info on the use of siege engines in the medievel period. I have heard the odd account of trebuget being used but no mention of ballista and onagers. And what about archiology, Has there been any substantial finds that tell of there use???

Archeological finds are usually just the missiles that were used .
Caerphilly Castle in Wales has several on display , plus it has 4 reconstructions (authentic as far as they have been able to ascertain) of medieval siege engines . If you are lucky you can visit on a day when they demonstrate these , if not then there is a video of each of them being fired into the moat/lake .
I think the last recorded use of a trebuchet was by the Turks(or whatever they were called at the time) against British ships in the Bosphorous . But that was not medieval and it wasn't a siege .