Most gates should be bashable-open rather than breakable,
Obviously the best solution. Make it a good, high number, something that could only really be reached by something BIG with a very high strength, magically enhanced to boot. Things I can think of off the top of my head that should be able to bash open gates:
Polymorphed Mage in Umber Hulk form (can tunnel through stone)
Shapechanged Mage or Druid with such forms as Dragon, Earth Elemental, etc. (the really big boys)
Ancient Dire Bears, Elementals, Pit Fiends (summoned or empathied)
Things like that. Some hulk of a PC (with an 18-20 STR) with an Enhanced Bull's and a Crowbar should NOT be able to bash them open (not enough mass behind the strength is a good enough reason). Is there any way to make them bashable, but so that Crowbars cannot be used on them? I think that would probably take care of the issue. Check the STR ratings of the various things I mentioned and set the Bash rating appropriately.
I mean...sure...given enough men with axes and fire, I can bring down any wooden gate. Thats why defenders pour boiling liquids and rain arrows on you while you are doing it.
The Garda need to step up their game.
Perhaps a script so that when the gates are bashed, Garda emerge from the towers, fill the walls and begin shooting. And if they are actually facing a truly supernatural creature (Dragon, Elemental, etc.) run screaming like girl-scouts.
There is some fun to be had with this....I am sure of it....