The Republics of Antiquity seem to have been relatively fragile creations, often victim to conquest from without or tyrants from within. What reasons might this have been so, and how could they have been more durable?
On the one hand, it would seem obvious that a city-state would be vulnerable to invaders, due to its small size. It would also seem to be an easy target for a tyrant; no matter how wide the influence of the City itself might be, control of the government of the city was all that was needed to control everything. Consider the civil wars of the Roman Republic, it was all ultimately a fight over who would control Rome itself.
On the other hand, we do not need to go too much later into history in order to witness city states that were relatively durable against both exterior and interior threats. Venice stood as an independent Republic for 1100 years, and none of its territory was near as important as Venice itself. Further, Rome itself is a testament of how durable a city-state Republic could be against internal threats, it lasted a full 400 years before civil war began to tear it apart.
Could it be then, that a network of cities could have been more durable? A federation or confederation of cities, able to withstand the fall of one or another? Possibly, but the only thing more ephemeral than a classical Republic was a classical League of Republics.