In September 2015 the Madrid City Council opened "Decide Madrid", a portal to discuss and decide the city model through debates and proposals. Debates were conceived as discussion threads opened and commented by any citizen. Proposals were designed to allow citizens to publish petitions, receive support from other citizens and finally run a public voting of the entire population. The present work is the analysis of Decide Madrid conducted within the EU-funded D-CENT project in collaboration with the Area of Citizen Participation, Transparency and Open Government of the City Council. The goal of the study is to reveal insights that provide feedback for the refinement of the platform. First we address the distribution of activity over time and observed peaks of activity. In the first week, users could only debate and, once proposals were available, activity focused on them rather than on debates. This might be explained because citizens were engaged in the possibility of achieving tangible political goals instead of just debating as they daily do in online social networks. When this pattern emerged, the managers indicated that debates would be improved by the participation of political representatives. Our results prove the success of this initiative. We also identify the community structure of the platform through social network analysis techniques. We find a well-connected structure without well-defined clusters of users: users were not strongly clustered around preferences. However, our analysis detects a cluster partially isolated from the main component of the network: a community of trolls mostly focused on goliardic actions. Finally, we assess a computational model to quantify the degree of deliberation given the structure of discussion threads, and present the results in an interactive visualization as a way to better understand how discussion builds Collective Intelligence.