Both sides then began building a series of walls. The Athenian circumvallation, known as ""the Circle"", was meant to blockade Syracuse from the rest of the island, while the Syracusans built a number of counter-walls from the city to their various forts. A force of 300 Athenians destroyed part of the first counter-wall, but the Syracusans began to build another one, this time with a ditch, blocking the Athenians from extending their wall to the sea. Another 300 Athenians attacked this wall and captured it, but were driven off by a Syracusan counter-attack in which Lamachus was killed, leaving only Nicias from the three original commanders. The Syracusans destroyed 300 m (1,000 feet) of the Athenian wall, but could not destroy the Circle, which was defended by Nicias. After Nicias defeated the attack, the Athenians finally extended their wall to the sea, completely blockading Syracuse by land, and their fleet entered the harbour to blockade them from sea. The Syracusans responded by removing Hermocrates and Sicanus as generals and replacing them with Heraclides, Eucles, and Tellias.