A city shall be viewed as a system composed of different subsystems essential for its functioning and performance, such as water, gas and electricity infrastructure, transportation, urban development and buildings, architecture, public services, healthcare, government and many others. These areas are typically considered to be independent and different municipality agencies, as well as diverse research teams consider them as separate topics, with one joint aim: to create a city which is efficient, livable and sustainable. Technological aspects are frequently presented as the key to the smart city development. This can result in overestimating of their influence and significance on the proposed solution. It is important to point out that using of a state-of the-art technological solution does not automatically mean it is the smart one.

Smart city (SC) is a complex and multicomponent system, and it is essential to describe it as well as all of its components using the same methodology. This is a key challenge and is not addressed by most of the existing projects. To describe complex system, so-called conceptual models describing formal and logical structure of the modeled system are used. Often a graphical form is used.

For example, Smart Governance, Smart Citizens or Smart Healthcare influence the city (or even region) as a whole. It cannot be implemented locally in a certain geographical area separately. Others, such as Smart Technology or Smart Mobility, can be linked to particular topological areas, such as streets, squares or parks. Even the general ones must be however taken into consideration when modeling Smart City.

Any city can be decomposed by repeating entities streets, parks or others. They can be clearly distinguished topologically and their specific objective functions can be defined. This diversification leads to the concept of building blocks introduced within this paper. Each building block is a natural entity with defined functionalities and clear interface to other building blocks.

The concept of building blocks allows us to create model of a city based on so-called Domain architecture. Domain architecture is a new term distinguishing the architecture specific for a particular building block. This significantly reduces the scope and thus the size of the problem. As will be demonstrated later, hugeness is one of the main risks and challenges of using general hierarchical architecture. This is demonstrated on the examples of so-called ITS architecture, where clear parallels to the SC modeling can be found. Similarly to SC, intelligent transport systems are also multi component and complex systems and are typically described by a conceptual model called ITS architecture. They are facing similar challenges such as the hugeness problem, questions about centralized versus decentralized intelligence or issues of cyber security.