Urban green areas minimize the negative effects of climatic change and improve the sustainability of historic cities. Despite this, green areas have traditionally been considered a threat to heritage buildings because they cause humidity changes, that accelerate degradation processes. Within this context, this study evaluates the trends in the inclusion of green areas in historic cities and the effects it causes on humidity and conservation of earthen fortifications. To achieve this goal, vegetative and humidity information has been obtained since 1985 from Landsat satellite images. The historical series of images has been statistically analysed in Google Earth Engine to obtain maps that show the means, 25th, and 75th percentiles of the variations registered in the last 35 years. The results allow visualizing spatial patterns and plotting the seasonal and monthly variations. In the decision-making process, the proposed method allows to monitor whether the presence of vegetation is an environmental degradation agent in the nearby earthen fortifications.The analysis of the historic fortified cities of Seville and Niebla (Spain) shows a gradual increase in green areas and an interest in locating them near the earthen fortifications. The impact on the fortifications is specific to each type of vegetation and can be positive or negative. In general, the low humidity registered indicates low danger, and the presence of green areas favours drying after heavy rains. This study suggests that increasing green spaces to historic cities does not necessarily endanger the preservation of earthen fortifications. Instead, managing both heritage sites and urban green areas together can encourage outdoor cultural activities, reduce the impacts of climate change, and enhance the sustainability of historic cities.