Download Rusted Pack File Manager https://github.com/Frodo45127/rpfm

Go to your workshop folder, usually found at C:\Program Files (x86)\Steam\steamapps\workshop\content and find your Attila folder. You'll have to search the numbered folders yourself to find the right file.

Open it in RPFM and set the application to Attila.

The values that are mainly of interest is Enemy Strength Modifier and Fortified Settlement Assault Strength Modifier.

The lower ESM, the more aggressive the AI gets in the field and the higher the FSASM the more aggressive the AI will be in siegeing your walled settlements.