Tip for fighting a superior enemy.

[Dubius Cato]

You can also try to place forts in any mountain pass, river ford, or other strategic place, or even build a limes with forts in any other map tile. Put some cheap (or understrength) unit in, and it will probably stop the AI for another turn. It gives you more time to scramble your forces, and maybe they call off the siege altogether and wander off. I always use this in the small accessible line in Lybia between the coast and the desert to stop the Ptolemaic stacks, you need a line of six or seven forts there, and another one with a defensive army that covers all of them.