Maybe they have the purple flame field going just underneath the roof, so Sarutobi and Orochimaru could levitate the tiles and use them for Shurikens like they did.

Oh, and the ANBU are not the sharpest knives in the drawer either. Hence, the one idiot who just jumped at the flaming forcefield and got killed.

The only ANBU that i have ever seen that has any kind of promise is the guy in the white cloak now.