A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrasts in Lorentz Microscopy. Various types of flux-closure micromagnetic states are evidenced and reconstructed using the Transport of Intensity Equation, from the simple Landau state with a single central Bloch domain wall to so-called diamond and multiple diamond states. The former was used to investigate various aspects of asymmetric Bloch domain walls. First, the experimental width of such complex walls was derived whose value and thickness-dependence was analysed with the help of numerical simulations. Moreover, quantitative datas on the magnetization inside the dot was retrieved and informations on the degrees of freedom of such walls was extracted. Finally, it is shown how the existence and the propagation of a surface vortex can be characterized and monitored. This demonstrates the ability to reach a magnetic sensitivity a priori hidden in Fresnel contrasts, based on an original image treatment and backed-up by the evaluation of contrasts obtained from micromagnetic simulations.