Xenophon Hellenica 7.2.5

[5]
  Once again, the Lacedaemonians and their allies were guarding Oneum, and the Thebans were approaching with the intention of crossing over the mountain. At this time, as the Arcadians and Eleans were marching through Nemea in order to effect a junction with the Thebans, exiles of the Phliasians made them an offer that if they would only put in an appearance to help their party, they would capture ¹ Phlius; and when this plan had been agreed upon, during the night the exiles and others with them, about six hundred in number, set themselves in ambush close under the wall with scaling ladders. Then as soon as the watchmen signalled from Tricaranum that enemies were approaching, and the city was giving its attention to these last, at this moment those who sought to betray the city signalled to the people in ambush to climb up.