I don't know how that happened. Maybe their aura was so strong that it... um... knocks over guard towers or something. Most likely, their fighting was so fierce that somehow ("offscreen" the supports of the tower were cut). Either way, it's kind of a cool scene.

And the fight continues

(Remember, it goes up to down on the right column, then the left)

GIICHI MIRRORSHADES RUSH ATTACK!!

Oh, I forgot. They're on a bridge. And Magatsu accidently cut the rope...

Magatsu falls, Giichi tosses his sword away...

...he hangs on to Giichi's foot, Giichi kicks him, he reaches down to stab Magatsu, but Magatsu traps his arm so that he isn't cut, and drops, falling a couple meters.