# Metaverse Architectures: A Comprehensive Systematic Review of Definitions and Frameworks
## 4 Discussion
### Metaverse Architecture
#### 4.2.3 Merits and Shortcomings of the Different Models  
Although the architectures share foundational elements, they exhibit significant divergence in their approaches to managing user interaction and experience. The three-layer architecture integrates interactivity, user environments, and economic activity within broad, consolidated layers, specifically, the interactivity layer and the ecosystem layer. In contrast, the four-layer architecture refines this structure by segregating these functions into more specialised components. For example, the interactivity layer is divided into a physical world layer, which manages real-world user inputs and sensor data, and a metaverse engine layer, which addresses immersive technologies, AI, and data synchronisation. This enhanced granularity supports modularity and scalability, facilitating more flexible system design and a clearer delineation of responsibilities. While the simplicity of the three-layer architecture may be advantageous in small-scale or closed environments, the expanded structure of the four-layer architecture offers greater adaptability in more complex use cases [105]. The four-layer architecture offers enhanced structural clarity and functional specialisation relative to the three-layer architecture; yet, both frameworks demonstrate considerable limits when utilised in extensive, socially integrated metaverse settings. A significant limitation is the lack of structural support for interoperability. Both the three-layer and four-layer architectures lack a defined architectural mechanism to enable cross-platform interaction. Rather, these models typically presume isolated or self-sufficient ecosystems. This design methodology restricts users' capacity to transfer assets, identities, or social connections between platforms, leading to disjointed experiences that impede the overarching goal of a  
Figure 8: Layer mapping between four-layer and five-layer architectures. cohesive metaverse [106]. This constraint highlights the essential requirement for architectural improvements that emphasise interoperability as a primary design element [107]. The five-layer approach mitigates several of these constraints through its more detailed and modular framework. By incorporating distinct layers, including the human society layer and the interconnected virtual worlds layer, it provides improved support for social complexity and cross-platform functioning. The interconnected virtual worlds layer explicitly promotes interoperability by allowing seamless transitions and data interchange among scattered sub-metaverses. This structural inclusion differs from the three-layer and four-layer, which presume isolated ecosystems and lack methods for identity transfer, asset continuity, or cross-platform connections. The separation of once-integrated functions into distinct, purpose-driven layers enhances modularity, scalability, and system clarity. A significant shortcoming in the three-, four-, and five-layer metaverse architectures is the lack of a dedicated governance layer. Although all three frameworks incorporate blockchain technology, primarily to support decentralised data integrity, digital ownership, and secure transactions, this integration serves as a technological enabl rather than a formal governance mechanism. Blockchain facilitates elements such as smart contracts and peer-to-peer exchanges, but it does not architecturally model processes like regulatory compliance, content moderation, or ethical oversight. As metaverse platforms continue to expand in both scope and societal influence, the absence of integrated governance mechanisms has emerged as a significant vulnerability [108]. Regulatory compliance, content moderation, and user rights management are often perceived as external or secondary concerns, resulting in a structural governance gap [109].