WASHINGTON - The Pakistani military was pounding a mud-walled fortress last night where a "very senior" Al Qaeda leader - possibly Osama Bin Laden's No. 2 - was holding out against a Pakistani siege. American intelligence officials were cautious about who might be inside the mountain stronghold, but Pakistani sources said they believe it was Bin Laden's top general, Dr. Ayman Al-Zawahiri, and he may be wounded. Al-Zawahiri, nearly always at Bin Laden's side during his taunting videotapes, helped create Al Qaeda and was considered the brains behind its operations. His capture or death would be a major blow to...