The StVO § 9 para. 1 prescribes: "Turning, turning and reversing.
(1) Whoever wants to turn must announce this in good time and clearly; The direction indicators are to be used. ... "
The StVO § 9a stipulates: "Roundabout.
(1) If, at the junction with a roundabout, sign 215 (roundabout) is located under sign 205 (grant the right of way), traffic on the roundabout has priority. When entering such a roundabout, the use of the direction indicator is prohibited. Within the roundabout, stopping on the road is forbidden.
(2) The center island of the roundabout may not be run over. Exceptions are vehicles that would otherwise not be possible to drive on the roundabout because of their dimensions. With them, the center island may be run over, if a threat to other road users is excluded. "
Im Kreisverkehr ist das Halten auf der Fahrbahn verboten
As described above, according to StVO § 9a paragraph 1 is prohibited holding on the road, if it is a roundabout.