 The Aabach leaves the lake between the villages of Boniswil and Seengen.Hallwyl Castle, one of the most important water-controlling "castles" in Switzerland, sits in the middle of the river on two artificial islands, about north of the northern end of the lake.On the southern outskirts of Lenzburg a tunnel takes much of the flow of the Aabach under the town to rejoin the river in Niederlenz township.