The simplifying assumption that the Central Business District is a point or featureless plain, is often made in urban economics. The main goal of this paper is to promote the relaxation of this hypothesis, with the support of empirical evidence, sustaining that the subsequent analysis can be supported on the agglomeration economies theoretical framework. For this purpose we?ve analysed the distribution of economic activity inside the city of Lisbon, using the employment data of ?Quadros de Pessoal do Ministério do Trabalho e Solidariedade? (Employment Datasheet of the Ministry for Social Security and Labour), for 1998. Proceeding with the address matching of this information to the street level in order to allow the use of GIS tools. From the analysis we emphasize the evidence of agglomeration economies inside the Central Business District, using as evidence the different levels of employment agglomeration that occur on different streets and testing the hypothesis that to streets with higher employment concentration correspond higher salary levels. Therefore we sustain that the CBD cannot be seen as a point or a featureless plain and furthermore we?ve verified the existence of an hierarchy inside the urban structure, which can be seen as a parallel of the rank-size rule. After this analysis we argue that the streets with larger employment volume display a structural role in the spatial distribution of economic activity. The final conclusion leads us to consider Lisbon as a squares network-city, where the axis are structural streets and the nodes are structural squares. This paper explores some of the results of the thesis presented for the master degree in Economics and Management of the Territory of The Technical University of Lisbon, made with the orientation of professor M. Brandão Alves. Keywords: Agglomeration Economies; CBD; Lisbon; Urban Economics JEL CLASSIFICATION: R12 e R14.