The lack of cooperation between stakeholders and coordination of departments, the absence of systems interoperability, and the resistance to change by policymakers represent some of the challenges that make the implementation of the Smart City a complicated task. Nevertheless, the literature is lacking a broader comprehension of the Smart City barriers to enable policymakers to design and execute their strategies. Therefore, this paper aims to find the (critical) barriers that have been associated with having a significant influence on the success of a Smart City's implementation and that are within the control of policymakers, based on the empirical knowledge and experience of experts. A study of the temporal aspect in which they can be overcome is also performed to allow better planning. Moreover, 50 barriers were aggregated and organized in eight distinct areas from a thematic analysis of an initial list of 114 barriers collected from the literature. Nine experts participated in the Delphi Analysis, which demonstrated that although Smart Cities are usually associated with technological and financial constraints, the 15 most critical barriers are mainly from the areas of Governance, Project, and Organization. The method combined a two-round survey with a focus group, using quantitative and qualitative approaches.