The authors investigate the potential of Mixed Reality (MR) games for team building and assessment. The AMELIO game was designed for a highly immersive MR lab. The game is a multi-player team challenge based on the concept of an escape room, staged in a space colony emergency situation. An explorative empirical pre-post measurement study was carried out to establish whether playing AMELIO influences team cohesiveness. Ten teams of three played AMELIO and filled out pre- and post-game questionnaires with validated measurements of team cohesiveness and mediating factors related to team composition, game experience and team dynamics. The findings show a positive and significant increase in team cohesiveness, with stronger effects for teams with lower pre-game familiarity. In terms of game experience and team dynamics, audio aesthetics and empathy proved to be significant mediating factors. This AIDS game validation and improvement, and understanding and guiding the team building process.