What is a breach during the medieval times?

Answer 1

When laying seige to a town the attackers would attempt to make a breach or hole in the city wall by throwing rocks at it or undermining it. The defenders would move more people there while they attempted to repair the breach and stop the enemy entering the city. It was a dangerous place to be. King Henry says in Shakespeare's Henry V Once more unto the breach, dear friends, once more;

Or close the wall up with our English dead. In other words - get in there, lads, death or glory!