A variety of methodologies to build secure systems have been proposed. However, most of them do not say much about how to evaluate the degree of security of their products. In fact, we have no generally-accepted ways to measure if the product of some methodology has reached some degree of security. However, if the system has been built with a methodology that uses patterns as artifacts, we believe that a simple evaluation is possible. We propose a metric for the security of systems that have been built using security patterns: We perform threat enumeration, we check if the patterns in the product have stopped the threats, and calculate the coverage of these threats by the patterns. We indicate how to take advantage of the Twin Peaks approach to arrive to a refined measure of security. In early work, we have proposed a secure systems development methodology that uses security patterns and we use it as example.