Escape Rooms are presently considered a very popular social entertainment activity, with increasing popularity in education field, since they are considered capable of stimulating the interest of players/students and enhancing learning. The combined game mechanics have led to blended forms of Escape Rooms, the Serious Escape Games (SEGs) and the hybrid type of Escape Rooms that uses Augmented Reality (AR)/Virtual Reality technology, a type that is expected to be widely used in the future. In the current study, the MillSecret is presented, a multi-player Serious Escape Game about local cultural heritage, where the players must solve a riddle about the cultural asset of watermills. MillSecret uses AR technology and it was designed to be conducted in the real-physical environment and in an informal educational context. The paper describes the game, its implementation, the playing process, and its evaluation, which aimed to study the feasibility of game conduction in outdoor settings and the views and experience of players with the game, the local cultural heritage and local history. Evaluation results reveal, among other findings, a very positive first feedback from players that allows us to further evolve the development of the game.