So you are attacked in your tiny 4 units garrison by half a stack or more and press Autocalc due to the horrible chances? Wrong. Wrong as (unfortunately) many of the Letīs Play Videos on you tube, which leave the impression you should defend the inner town circle. For the tiny morale boost.
Instead you look at the 1 tower which is the easiest to defend due to limited space. Make sure you can set up a reliable and hard or impossible to flank defense outside of its yellow influence ring, otherwise the sheer number of enemy troops disables it quickly. Position your 1 lone light cavalry somewhere else and hidden from the enemy view.
Then just wait for the enemy, never leave defensive position and let the tower do its work. Your cavalry waits until the enemy is set up, swings around the town and moves against the unprotected archers/slingers. 1 light cav is good for 4 archers (yup), if you donīt chase routing units but instead continue attack. Chase routers only after all attacked units rout until numbers are below 20 (means destruction for a typical 120 size infantry unit). After that and if your tower defense still holds - attack the enemy in the rear. Move in, out after a few seconds of contact, in again.
Set up this way you may defeat up to triple the units of an attacker (which can be its stack size), especially if its general dies early in spears or tower fire. In any case you should be able to inflict way above 700 casualties to an infantry heavy attacker, including some annihilated units. Autocalc would give you may be 150 ...
Funny little anecdote: In my first defense in Attila ever (me ERE against Visigoths) I did it wrong. 2 spear infantry were set up for plaza defence, cavalry was successfully destroying archers, but it was over after my "general" died. Well, not really. I had for a reason of weird thinking a heavy sword infantry near a tower in light forest, thus hidden. The attacker never spotted it so I spent a minute watching the AI massing troops somewhere in the town and doing nothing. I got bored and my infantry showed up, then raced back to the tower. 2 cavalry came after me first, ran into tower fire and then a roman infantry in defensive testudo. Boom - 1 more cav annihilated, the other one routed :-).
Since then I never did autocalc on defense. You can hold your worst possible enemy stack for Romans (Mixed Hun stack of year 420 with 2 large onagers and some competent infantry) with just 7 heavy infantry in a level 2 town. Battle difficulty VH.
Regards,
Thorsten