Something similar happened to me sieging the city just north of Arimium.
My forces were lead by a rookie general with little command, but consisted of huge amounts of troops (3 hastati, 3 principes, 4 triarii, three equites, my general, 3 ascenci, and three leves), fighting barbarians with a moderate amount of troops (2 Naked Fanatics, 2 swordsmen, some spearmen, and a heavy cavalry plus their general).
They rushed out of the gate, but only with one of the Swordsmen, whom my cavalry immediately slaughtered. They sent another one out, and agian my cavalry slaughtered them, but took a few casualties so I retreated.
Then they rushed all out with no strategy, at which point they met a huge hail of fire from all my units. They still charged on and eventuallly their battle line and mine of hastati and Principes merged. My cavalry charged theirs and killed off their heavy cavalry and was fighting their general, at which point they were reinforced with 2 units of Triarii. The other two went around the back and flanked the Gestarnae. Then I charged in the leves to support the rest of the battle line, and once their general was dead I sent the two Triarii who had dealt with him to destroy the Gestarnae.
To my great surprise their units did not run, and the Gestarnae actually killed off two units of Principes and four units of Triarii on their own. At this point most of my army routed and I had to give up the battle, though I fought to the last man.
My general later took a fully reinforced Legion of 6 Principes, 4 Triarii, 3 Ascenci, 3 Leves, and 3 Equites to take revenge on the city. They decided to simply surrender and offer the keys to the city to my general. For their submision I decided to merely enslave the populace, rather than Exterminate them as I wanted to.
btw, I don't know why you are complaining about Hastati. In my experience they are even more useful than Principes. I don't know why, but I find my Hastati rout less, after more time/losses, and handing out more losses to the enemy than my Principes do.
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