In the last decade, the use of steel exoskeletons as an alternative to seismic retrofit of existing structures received great attention due to their non-invasive and time-efficient nature. Furthermore, it represents the preferred solution in cases where the use of the existing building cannot be interrupted during the intervention, or when a limited level of knowledge of the existing building is available and high seismic activity is expected. Nowadays, the proposed design approaches mainly rely on simplified models employing single or multi-degree-of-freedom systems with lumped masses to determine optimal overall values for exoskeletons' stiffness, mass and/or damping. Even though the benefits derived by these methodologies were largely proven, further investigations are required to fully understand the influence of mechanical and geometrical properties on the outcomes.