- Defender  1 to 2  .  Attackers need 2x to break even w/ lvl 3 walls.   
- I tested 1100 troops vs 2200 and fended off the invaders.  (defender start out 400 and attacker 800+)  
- Siege battles last 20-35 minutes.  The battle above last 35mins
- I changed a few value in the 2 files for myself to play. Nothing crazy. You can change it if you don't want to download and extract. 
- I change it so The batteringram and siege towers move a little bit slower to stay true to velocity and up hills ..etc. In vanilla the troops push them like they're running marathon up hill pushing a foam cart.
- Batteringrams will hit the door  80-130 times about 6 - 8 minutes (depends on wall levels) when start ramming the outer doors. 
- I tested this with 75+ mods . Super crazy fun. =)
- I recommend Realistic Battle Mod https://www.nexusmods.com/mountandblade2bannerlord/mods/79
especially Realistic Battle AI that give a complete reinforcements for both sides.

I have included my mod list loading orders in Pictures section

----!!! WARNING !!!---

Backup your 2 files to a folder (any folder) so you can change it back later if you don't like it.
It has nothing to do w/ save files so it's save compatible.

-The 2 files are in bannerlord native directory. Mine are.  

D:\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules\Native\Prefabs\props_siege_batteringrams  
D:\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules\Native\Prefabs\props_siege_siegetowers 

---!!! EXTRACT!!!------------------------------- to this directory mine is on Drive D:
the file to your bannerlord directory,   D:\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules\Native\

I modified 
D:\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules\Native\Prefabs\props_siege_batteringrams
using notepad. These values are right in the beginning so you don't have to search.  I only change 3 values down below . you can change 

<scripts>
<script name="BatteringRam">
<variables>
<variable name="IdleAnimation" value="batteringram_idle"/>
<variable name="KnockAnimation" value="batteringram_fire"/>
<variable name="KnockSlowerAnimation" value="batteringram_fire_weak"/>
<variable name="KnockSlowestAnimation" value="batteringram_fire_weakest"/>
<variable name="KnockAnimationHitProgress" value="0.560"/>
<variable name="KnockSlowerAnimationHitProgress" value="0.590"/>
<variable name="KnockSlowestAnimationHitProgress" value="0.610"/>
<variable name="RoofTag" value="roof"/>
<variable name="GhostEntityMove" value="false"/>
<variable name="GhostEntitySpeedMultiplier" value="0.700"/>  (I changed value="1.000" to 0.700 . This is how fast the battergram move when touch by troops) 
<variable name="WheelDiameter" value="1.300"/>
<variable name="GateNavMeshId" value="50"/>
<variable name="DisabledNavMeshID" value="40"/>
<variable name="NavMeshIdToDisableOnDestination" value="-1"/>
<variable name="MinSpeed" value="0.500"/>
<variable name="MaxSpeed" value="0.700"/>   (I change value='1.000" to "0.700" to make it move 30% slower)  
<variable name="DamageMultiplier" value="25.000"/>  (I change value="300.000" to 25.000.  This is how hard the batteringram hit the doors .  You can change it one hit by change value to lot higher or more hits if change it lower)

I modified 6 values to make siege towers move slower. 
D:\Steam\steamapps\common\Mount & Blade II Bannerlord\Modules\Native\Prefabs\props_siege_siegetowers 

<variable name="MinSpeed" value="0.400"/>  (I changed these value="0.500"  to 0.400 ; there are 3 of these lines. use search to change all 3)
<variable name="MaxSpeed" value="0.600"/> (changed value="1.000" to 0.600 ; change these 3 lines right below minspeed . This is how fast the siege tower move. you can change it higher to run super fast or lower to move slower)

============================================================================

If you're messing up your game.  you can go to steam> game library> right click Bannerlord > Properties > Local Files> Verify integrity of game files .
To restore bannerlord to default. 

I'm still figuring out how to make a proper mod.