Why were British soldiers guarding the North Bridge?
General Gage, in his orders to Lt. Colonel Smith, commander of the Britsh expedition to Concord, directed him to take control of the two bridges in town, the South Bridge and the North Bridge. "You will observe...that it will be necessary to secure the two bridges as soon as possible..."
Securing the bridges was necessary to prevent rebels from slipping across from remote parts of town to threaten the mission. Also, Lt. Colonel Smith sent seven companies across the North Bridge with orders to search for supplies and artillery known to be hidden at Barrett's farm, about a mile west of the bridge.
At that time, the colonists occupied the high ground overlooking the bridge. If they were to swoop down and take the bridge, the British soldiers at Barrett's farm would be cut off. Therefore, the British left three companies (about 96 men) at the bridge to guard it.