In recent years, plenty of novel educational tools using mobile augmented reality (mobile AR) have been developed, altering the way students interact with a cultural heritage environment. Nevertheless, the relevant literature still lacks studies exploring the usability, effectiveness and acceptance of the technology by users, and there is still need for in-depth research to better understand the way students interact with mobile AR interfaces in this context. This study explores the use of mobile-AR technology in an open-air archaeological site and the results of this study are aimed at increasing the current literature on the digital encahcement of open spaces of cultural interest. This study also seeks to provide cultural heritage administrators and teachers with some key elements and factors that should be taken into account when deciding on the methods used to guide students to archaeological sites to develop new and more effective digital approach strategies. The paper describes the design, development and evaluation of the pilot application "Oracle of Delphi app", an educational augmented reality application for the Delphi outdoor archaeological site. It is a location-aware portable game aimed at secondary school students and aims to involve them in a playful exploration of the area through which they will learn about the most important monuments of the sacred site of Delphi and discover their history. This study is the only attempt so far to create an educational game using mobileAR for a guided tour of the archaeological site of Delphi which is then tested and evaluated by the students.