Progetto Ustica is an experiment in "civically-engaged game design" and addresses the Ustica Massacre ("Strage di Ustica"), where 81 people lost their lives as an air-to-air missile hit a civilian aircraft in 1980. Progetto Ustica has been developed as part of an Action Research effort with the objective of preserving and transmitting the historical memory of the event. With this post-mortem essay, we reflect on the game design challenges that Progetto Ustica faced, we introduce different conceptualizations of "memory" emerging from our design practice, and we synthesize some lessons learned (implications for design) towards other "civically-engaged games" for socio-cultural heritage.