Undercurrents of discord between the Salian royal family and the Saxons already existed under Henry's father, Emperor Henry III. This may have been primarily due to his Rhenish Franconian origin as well as his numerous stays in the Imperial Palace of Goslar, which were associated with a disproportionately high economic burden on the surrounding population. With the accession of Henry IV in 1065 this conflict intensified, as Henry made demands on numerous Imperial domains (Reichsgüter) in the centre of the Saxon heartland around the Harz mountains—especially the silver mines of Rammelsberg. To secure these estates he initiated a castle building programme, erecting numerous fortresses along the range, the most prominent being the Harzburg. This was perceived as a threat by the Saxons. In addition, these castles were staffed with ministeriales of Swabian origin, who frequently plundered the Saxon population to make up for their lack of income..