Interestingly, sun tzu says in art of war to never completely surround your enemy, always give them a break out direction because if they know they are trapped they will fight hard.
One of ceasars great victories in gaul, the battle of alesia, he was surrounding a town and then a relief army showed up and surrounded him. He built like a double fort, one side facing the town he was besieging and one facing the relief force.
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