From a logistic standpoint, this is heart of Paris (or according to Zola, the stomach) : the place where all the railways cross, where masses of people flow in and out day and night. It was and is once again a place of gigantic urban planning and architectural experimentation. From their role as the central marketplace beginning some 800 years ago when Philippe August built the initial structure, Les Halles were transformed from the 1970's into a vast public space for leisure and shopping. The Pompidou Modern Art Center (Centre Pompidou or Beaubourg), once perceived as a monstrosity with its metal guts displayed on the outside, has become as popular a place to visit as the Eiffel tower. Today, the neighborhood is destined for yet another vast and futuristic project to be completed in 2016. Until then, the neighborhood will continue to fascinate with some of the city's oldest streets and most amazing stories. For example, Henri VI was assassinated at 11, rue de la Ferronnerie. The pictoresque rue Quincampoix marks the downfall of the Scottish banker, named Law, who did just a good a job of bringing on financial ruin as did Bernard Madoff.
Incidentally, for the purists, Beaubourg borders the 1st but is part of the 4th.