Assessment of a web-delivered game designed to engage a target audience of middle-school girls in virtual collaborative physics projects to improve understanding of the nature of science, technology, engineering, and mathematics (STEM) disciplines and careers. The intent is to help students understand introductory physics, and the first prototype game experience explores the domain of simple machines.  The experience is goal-driven while also emphasizing collaborative and exploratory play styles.  The assessment objectives address the effectiveness of the gameplay experience to engage the target audience as well as the gameplay's ability to achieve intended educational outcomes.  The analysis includes the development and deployment of instruments for technical literacy as well as pre- and post-assessment measures of STEM efficacy and simple machine identification and use.  In addition to this analysis, exploration of data gathered from direct instrumentation of the game engine as well as qualitative data from game screen and video play capture are essential for assessing the overall play experience.  This research was funded through the NSF Small Business Innovation Research (SBIR) Phase I program.