The castle was built between 1220 and 1230 by Vogt Rudolf von Rapperswil, who until then had had a modest residence on the other side of the Obersee in Alt-Rapperswil. From his new position, Rudolf, who was appointed Count shortly thereafter, was able to easily monitor the waterway from Zurich to the Bündner passes as well as the flow of pilgrims to Einsiedeln Abbey and the traffic crossing the narrow lake at the Seedamm. The town of Rapperswil soon developed at the foot of the castle.
The castle, rebuilt by Albrecht II, forms an almost equilateral triangle, with each corner reinforced with a tower. The highest tower in the south-west is called the Bergfried or also Gügeliturm and was for a long time the seat of the high guard, who warned the town and castle residents of approaching danger or flames with horn blasts. The pentagonal time tower in the east houses three bells and, in addition to a sundial, two large dials show the time. The large castle building is located between these two towers in a south-easterly position. In addition, battlements lead from the two corner towers to the third tower in the north-west, the Powder Tower. Today's Bleulerhaus in the former Burggasse and the so-called Breny Tower served as outbuildings of the castle.
Source: wikipedia