Stall for time. Use boats to interfere with the enemy transporting troops by sea. Use diplomats to secure peace treaties with your other neighbors, and sell maps to raise money. Don't use field troops for garrison duty; use peasants and town watch, to save money. Let the enemy siege your towns, if you have to. It will tie up an army for several turns. If you can catch the enemy broken up, destroy them piecemeal. Try and encourage their neighbors to attack them. If worse comes to worse, destroy every building you can in a threatened town, and pull out of it before they can get to it to take it. You can even gift or sell it to another faction after you've done so. If it revolts, it'll come back to you or turn rebel, with a big rebel army in it that will delay your foes.