In 2017, the United Nations adopted a global Sustainable Development Goals (SDG) indicator framework, calling on member countries to collect complementary national and regional indicators. Cities are crucial to channelling efforts towards sustainability through the use of these indicators. They provide an integrated approach to the city situation monitoring sustainability. However, more research is needed to understand how to adapt the goals, targets and indicators to specific municipal contexts. In 2020, the Spanish Sustainable Development Solutions Network launched the 2nd edition of the Spanish Cities Index. A set of 106 indicators allows for monitoring the implementation of the SDGs at the local level for Spanish cities. The objective is to perform a statistical audit to evaluate the consistency of the indicators and the impact of modelling assumptions on the result. The methodology used is an adaptation of the Handbook on Constructing Composite Indicator prepared by the European Commission. The indicator system is well balanced and covers the essential areas of the Sustainable Development Goals. The Spanish ranking is robust enough among the alternatives evaluated. However, some improvements are possible in the selection of indicators, e.g., removing redundant indicators and regional data. Finally, it is recommended to weigh goals based on municipal responsibility to adjust the results to the Spanish municipal context.