If you are starving because your army is too big, consider launching pyrhhic attacks to get rid of your excess troops. In that time, you zap them with your reinforcements. They won't move from it until they can defend it properly, which takes quite a few turns.

Simply divert the reinforcements to kill the enemy expeditionary force. Either way, you are creating reinforcements and moving them up the line. Two things can be happening: either you are swimming in gold and mana because you've taken so many cities, or you are starving because your offensive army is too big. Either you are right on the enemy's crown city when this happens, or you are engaging their main army close to the heart of their empire. It will send smaller expeditionary armies to take empty cities. Usually early on, the enemy will cease considering sending large armies to outflank your main army and will remain largely defensive. As long as you keep killing enemies every turn and live, you'll win by attrition until you reach the tipping point. Unless I am completely forced to halt an offensive (it happens), I won't waste time or gold on walls, roads, or defending armies.

The best way to defend your towns is to keep taking the enemy towns until they stop fielding units they can use against you.

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