Destroying defending units

[Freyr Oakenshield]

What's your opinion on the dificulty of dealing with units on defence?

I mean I have the impression that there must be some sort of error in the engine since it's extremely hard (impossible?) to do that. For example, I'm playing FitE with an opponent at the moment. It's turn 30 and they still haven't taken Oddessa. And I don't think it has anything to do with player skills. I remember having the same problems when playing FitE in the past against someone else and trying to deal with German entenched units in 1942. Besides, the bloody arty doesn't seem to help at all... Or at least I can't see any significant difference... It's a bit like fortified unites were invincible...

What's your impressions?

edit:

Or maybe it's just the FitE scenario and its settings; frankly, I haven't been playing anything else for a long time...