Usually when im playing julii there is lots of rebels in northern Italy. So if you see 2 Peasant units take 1 cavalry unit and have a big party whit their flesh.
If you see 7 peasant units take 1good cavalry unit and do the same. Soon you will have lots of good cavalry units.
Ambushes are also good way to increase your experience. I really dont know why but I have seen multiple times that even whit my virgin units in their first battle they tend to get 2 experience points during ambush.
Also it should be said that size cap between your and enemy unit also has major efekt. So if your unit has only 12 soldiers left it tends to get easier expirience. So dont get too often retraining your units if you want them whit high expirience levels.Just dont get them killed for good because of this.
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