An optimization procedure to design the heliostat layout in Solar Tower plants is introduced in the present paper. Whilst typically the mirror layout generation aims to maximize the annual power production, the model presented in this work determines the optimal heliostat distribution when the overall efficiency is maximized (and the LCOE is minimized) for specific periods of the year. This is crucial for CSP plants operating in island mode or within isolated grids, where the power demand is variable throughout the year and massively concentrated in certain months, and the backup units have limited capacity. The study focuses on a solar field corresponding to a nominal power of 5 MWe. The optimization process allowed to determine radial spacing of the mirror rows, tower height and position of each single heliostat for three different design periods. The resulting layouts exhibit a maximum annual field efficiency ranging between 64.78% and 65.77%. Introducing the economic evaluation, 18.43 cent USD/kWh is the best LCOE value achieved for the layouts designed for March conditions. Furthermore, the study shows that the fields optimized for June achieve the highest peak efficiency values, but the annual yield is better if March is considered as design period. (c) 2020 Elsevier Ltd. All rights reserved.