A combination of quantitative microscopy (petrographic image analysis) and mercury intrusion porosimetry (pore space textural analysis) allowed for a clear distinction of natural stone and mortar samples from two separate construction periods of the Riga Castle (Latvia) in the 13th and 14th c. For the task of the separation of construction phases, pore space textural characteristics seem to be superior over qualitative (descriptive) characteristics of the rock fabrics of natural building stone - dolostone, whilst quantitative microstructural data (grain size characteristics) are more useful than pore space textural characterisation of mortars.