For the evaluation of the influence of wall exposure and lithology on stone decay an open-air experimental site (a fortress) has been selected. The nearly 150 years-old Citadella fortress is located near the city centre of Budapest, where high SO2 concentrations and settling dust depositions are recorded. The building blocks are from soft (compressive strength is 7 MPa), light (density is 1 700 kg/m3) and porous (porosity is 27%) oolitic limestones. The detailed survey revealed that crusts (white and black) and mechanical breakdown features such as scaling, flaking, blistering and granular disintegration are common weathering features. The types of decay forms are markedly different on exposed and sheltered walls. Sheltered walls show significant blackening (up to 100%) while wind and rain exposed walls display mostly white crusts and associated scales, flakes and blisters. The most severe decay is recorded on exposed walls where granular disintegration leads to a surface loss of maximum of 5 cm. These blocks are found at the lowermost parts of the walls or at edges. The mineralogy of the crusts on sheltered surfaces (black crusts) and exposed blocks (white crusts) is also different. The first ones are very rich in gypsum while in white crusts calcite is the primary mineral and only a smaller amount of gypsum has been detected. The Miocene oolitic limestone shows similar weathering features to other oolitic limestines such as Monks Park limestone but due to its lower strength and higher effective porosity it is more prone to granular disintegration.