And I mean when lets say that city has a potential of getting siege or on the front line not if its far from danger

For me its

This is in total war medieval 2 since its almost the only one I play

Small settlements (villages that has no walls or have tier 1-2 walls)2 spear men militia if able one calv unit and if they have tier 2 walls 2 archers units.

Medium villages(tier3 or higher walls)4 militia men one or 2 siege machine 1-2 pike men if able 2-3heavy calv and as much archers (prob 5-4) specifically cross bows they just shred infantry and calv,and or archers who has decent melee capability because they just suck sometimes at following orders.

Capital citadesls or huge citys.6 militia men-mid or 4 high tier dismounted knights.2 2-handed units 4 heavy calv ,2-3 siege machines,2 pikes,6 crossbows.

This is my defense build(prob why I'm always so broke) what's yours