We discuss a few old results on the spectrum of the 3d Ising model and on the behaviour of fluid interfaces in the low temperature phase of the model which strongly disagree with the predictions of standard perturbative calculations. Performing a duality transformation (i.e. mapping the 3d Ising model into a Z_2 gauge theory) we show that these apparently puzzling results are signatures of an underlying effective string description of the $Z_2$ gauge model. We then discuss a few recent advances which drastically changed our understanding of these effective string theories and allow to precisely fit both the interface free energy and the low temperature spectrum of the Ising model.