The desire to secure our cities against threat is likely to have some profound effects on city form. Will we recreate the walled city with electronic and concrete barriers or are there prospects to do something much smarter? This presentation will consider two different ways of thinking which apply at once to models of cities as well as social, political and commercial organizations-the 'tree-like' hierarchy and the 'web-like' network. These are forms of thinking which have informed conceptions of the city from the early work of Jacobs and Alexander to recent conceptions of 'smooth' space and rhizomatic spatial practices. Both networks and hierarchies have their advantages and dangers. Cities have always embodied both hierarchies of identifiable control and networks of exchange, resistance and violence. A first response to September 11 is to call upon stronger hierarchies of control. But any such retreat to a 'walled city' mentality will kill civil urban life more effectively than violence. Is there a more sophisticated approach through understanding the potency and resilience of urban networks-meeting terror on its own turf and starving it of targets in multiplicitous cities of difference?