To those always crashing at the 5 trebuchets:
Next time reload the game, retreat, close the browser. Then try Chrome.
Seriously, no kidding. I just solved a firefox user's ticket on de with this very problem, and since there's no solution yet, I had him try Chrome for its very low crashing rate. He was a little more surprised than me that it worked, but it did indeed.
edit: Gotta revoke this, seems it worked coincidentially due to little variations in movement... It's the old trebuchet-bug, especially if they're encountered with shooters.