Jamieson, Fausset and Brown on Joshua 10:2

2. they feared greatly--The dread inspired by the rapid conquests of the Israelites had been immensely increased by the fact of a state so populous and so strong as Gibeon having found it expedient to submit to the power and the terms of the invaders.

as one of the royal cities--Although itself a republic (Jos 9:3), it was large and well-fortified, like those places in which the chiefs of the country usually established their residence.