I have now managed to do that... and completed the other campaign mission available too! The way to do it is to block in your marksmen so they cannot be gated... and DEFEND in the back corner like hell... just keep concentrating your fire on the devils... get your griffins to battle dive the succubusses, concentrate your ranged fire on the devils then the pit lords, then when they are gone (your griffins will distract the others when the land after killing the succubusses (or is the plural succubi?)...
ALSO!!!! and you MUST listen to this one... make sure that the hero you use to defend is Isabel, NOT Godric. She is so much better you would not believe!
You may find it easier to kill the prison guards with Godric FIRST then take the city... then nip round the corner with isabella, kill the dragons and get the gold mine, (you WILL need the gold, I promise...) then in the city get as many unit producing buildings built as possible.
Expect another attack about 10 days after the first... Again use isabella to defend (you may get away with getting him BEFORE he gets to the city)
After that... you should find that although from time to time you get attacked, you will by then (hopefully) be able to build enough units in your city that you can defend it with Godric AND have a decent-sized army out and about with Isabel