Off-Table Rules for Siege from Tony Bath
Part of the Project HexTrail research
Setting Up a Wargames Campaign by Tony Bath contains a section on resolving sieges without using minis on the table.
It is quite simple, consisting of just a few die rolls. In fact, it is so simple that the siege might be over in no time, if the Garrison Commander is weak or is liable to succumb to Treachery!
Here is a short podcast video reviewing the process:
