A material-technical research has been carried out on the building materials of the archaeological site Coudenberg at Brussels. The salt damage phenomena are investigated in relation to the actual climate as basis for an environmental management. It turned out that a climate in which the relative humidity is constantly stabilized between 50 and 60% could reduce the salt damage on the building materials. Alternatively, a salt extraction procedure according to modalities defined through a test application is recommended. The salt crystallization pressures provoked damage to the building materials generally in the form of powdering. This surface alteration was visualized by hardness profiles obtained by means of a drilling device. Consolidation tests proved to have a positive effect, except for the bricks showing severe crack formation as a consequence of the vibrations resulting from passing trams and busses on the square above this part of the archaeological site. The problematic of the humid conditions of underground built constructions, and consequently biological contamination, is discussed.