Isenstar Lake is at the north of Alagaesia. The city of Gil'ead rests on it's southern shores, while the immense forest of Du Weldenvarden covers much of it's nothern shore. The waters are full of fish, so in the daytime people can see the fisherman fishing in the clear waters. There is also a small island at the center of the Lake, which houses the Dragon Rider Ruins. Once a base for Dragon Riders long ago, even before Eragon, the castle has now fallen into extreme decay and ruination. At Gil'ead, one can take a ride on the Captain Sweeny's ship, the Dragon Heart, to the island for furthur investigation of the ruins.
Isenstar Lake

Isenstar Lake on a map of Alagaesia (The Red dot).