Two-dimensional discrete dislocation dynamics simulations are used to model the plastic deformation of an fcc metallic material containing large densities of defects. An obstacle model is proposed, based on the line tension concept. Increasing yield strength and hardening are obtained when the obstacle density is increased and destroyable junctions are included. A high dislocation source density is used to obtain a good dissemination of dislocations. Over 30% of the total density is stored as junctions. Slip is shown to be localized within a few intense slip bands, whatever the obstacle density. This localization is quantified as a function of the density of obstacles.