## 2 Position of This Work Compared to the Current State of the Art  
The approach proposed in this paper is based on the four pillars, namely: User Localization, Multimodal Interaction, User Understanding and Gamification, since, as we discussed in the Introduction, we conceive cultural heritage as an interactive intelligent environment able to localize and identify the user and interact with him engagingly and amusingly. The literature proposes many papers dealing with the improvement of cultural heritage fruition by innovative technologies; however, most of them consider an update of existing solutions employing one of the four pillars proposed here. We have observed great interest in augmented and virtual reality technologies; several surveys, such as [2; 9], focus on this specific technological aspect, highlighting its importance to ensure both amusement and accessibility (especially when physical access is constrained), discussing future research directions and comparing different categories of immersive reality. Other surveys focus on the methodologies of machine learning more exploited in cultural heritage [10], provide an overview of existing 3D repositories [11], or outline museum gaming technologies and applications [12; 13]. There is a review of interactive systems for cultural heritage [14], but the works are analyzed from the perspective of their empirical evaluation, that is, the degree to which the system satisfies user goals and expectations. To the best of our knowledge, no other survey analyzed the Cultural Heritage literature considering all together these four aspects that in our opinion are altogether significant to develop an intelligent and engaging fruition to cultural heritage. Moreover, a discussion of one of these pillars, inserted in a broader discussion including the other three, in our opinion, can be of greater inspiration for a more effective, multi-faceted approach. As an example, a reader interested in Gamification could be inspired by the discussion about the other pillars, considering some of the reviewed technologies and methodologies for defining new gaming strategies. We believe that a four-pillars vision contributes to covering, from a holistic perspective, the main issues of cultural heritage fruition--user localization, accessibility through different modalities of interaction, adaptation to user profile and interests, and the provision of an amusing experience. The reason for doing this review came from some of our ongoing experiences in the field of innovative content fruition and, in particular, in the context of cultural heritage (the experiences will be discussed in Section 7). We aimed to address an overview of the state-of-the-art and, in consideration of the fact that rapidly evolving technologies can be leveraged in this context, we were unable to find a sufficiently recent work that was useful for our purposes. We concluded that a more updated review was important. Just to give an example, an important aspect in museum navigation and in contextualized information provision is that of localization, an area where significant changes are observed over a short period. Many interesting technologies are emerging and evolving separately, we assumed that this work would be of interest to those like us who wanted to develop something dedicated in the field, figuring out how to bring different technologies and methodologies together. We planned this review by working according to a well-established procedure. The first step was to analyze the main issues to be addressed in this context, leading to the identification of the four pillars mentioned above.We then used the most important research repositories to consider. We analyzed works between 2017-2021, mainly concerning the more methodological aspects, with some exceptions for a few approaches that, although older, we found inspirational, and for some technological aspects where we considered a reduced temporal range.