Through more exact climate measurement, including measuring the building’s air exchange rate (tracer gas and a-value measurements) and CO2 measurements, an important question for historic buildings with a large number of visitors was investigated: Whether, as is generally accepted, climate-caused damages to furnishings and room surfaces are caused by the direct effects of visitors (up to 10,000 visitors per day). It was possible to demonstrate that uncontrolled natural ventilation, which occurred through unsealed windows and outer doors together with historical ventilation shafts, resulted in one air exchange per hour at low wind levels, which is conservationally harmless and guaranteed a satisfactory air exchange rate even for large numbers of visitors. Stronger wind levels, however, and arbitrary opening of windows by personnel, particularly when the doors to stairwells were open, led to air exchange rates of up to fifteen times per hour, so that significant climate changes occurred, whose result are the observed damages. Our complete concept for prevention, realized in a first renovation phase, combines tempering of up to 5-m high rooms through wall-base heating pipes (with which a minimum room temperature of 17°C could be maintained and the wall humidity could be reduced from almost 10% to 1.5%) with “controlled natural ventilation.” Ventilation is controlled using the simplest possible measures: sealing windows, reactivating the existing exhaust shafts in the room walls, adding fire dampers (for cutting off the air flow) as well as airflow-enhancing fans (whose electrical current production due to autorotation shows on a display whether they need to be activated) to the shafts, and reactivating the fresh air shafts in the stairwells, whose fresh air comes from an underground “air fountain” (a rediscovered, approximately 270-m long, former watering system), which thanks to its constant temperature can be used for cooling and dehumidification of the fresh air in summer and warming and humidification in winter. This concept, with which satisfactory room climate conditions are reached for the largest part of the year, could be optimized with simple additions such as a heat exchanger (to be able to avoid air humidification due to the earth channel in midsummer) and a bypass (for direct fresh air intake).