Sar Veak, who guarded the administrative hub and residence called K-1, supported statements from previous witnesses concerning the frequent visits of the three accused traitors, Nuon Chea, Ieng Sary and Khieu Samphan.
"They came to the place very often," Veak said.
The Phnom Penh Post said the three men are accused of crimes against humanity.
Veak, who testified Tuesday, said he did not know what the men discussed in the meetings, and that the information he heard came from higher ranking officials, such as Oeun Tan, who had also testified about the traitors.
Sary and Chea remained in their holding cells during Veak's testimony due to poor health.
Notable deaths of 2014 [PHOTOS]