We investigate the mechanism of polyelectrolyte multilayer (PEM) assembly in nanoporous templates with a view to synthesizing nanotubes or nanowires under optimal conditions. For this purpose, we focus on the effect of parameters related to the geometrical constraints (pore diameter), the size of the macromolecules (their molar mass and the ionic strength), and the interaction between the pore walls and the adsorbed chains (modulated by the ionic strength). Our results reveal the existence of two regimes in the mechanism of PEM growth: (i) the first regime is comparable to that observed on flat substrates, including the influence of ionic strength and (ii) the second regime, which is slower in terms of kinetics, results from the interconnection established between polyelectrolyte chains across the pores and leads to the formation of a dense gel. As a consequence, the diffusion of polyelectrolytes in nanopores becomes the controlling factor of PEM growth in this second regime. The dense gel, owing to its peculiar structure, enhances the formation of nanowires or of partially occluded nanotubes in some cases, depending on initial pore dimensions.