I recently bought E:TW as well and yes there are some irritating problems with it. But the company was 'aledgely' put into an impossible situation (or just wanted to rip you off depending on which conspiracy theory your currenty subscribed to ) and released way too early.

Overall i enjoy the game, it's quick, simple and easy (relatively) and an enjoyable distraction from Europa Universalis III, so i'm more than happy to put it on the shelf for a little bit and get it back out in a month or so when the next (in a they promise to be a long running series) set of bug fixes and updates.

But to answer your question, yes if you have fewer troops than walls then the fort is basically pointless, in fact it's a major hindrance. Even with a full army in the largest fort available, you don't have enough troops to man all the walls ..... they'll sort it out no doubt. So basically abandon the fort and set up defensive positions next to it.

The one I'm waiting for most is the 64-bit friendly executable. On Grand Campaign it used to crash every second battle from running out of virtual memory (even though I have a 1gb graphics card and 12gb ram, doesn't mean squat to a 32bit exe). Found a post with a tool to change the executable header to be large-address friendly so it uses a bit more and it got a lot smoother. I don't even care about the AI wall climbing funkyness as long as I can play without crashing/restarting every 30 minutes (or worse yet when it crashes 30 minutes into a battle!).

I'm been playing various games waiting for the problems of E:TW to be sorted out (EU3, Elven Legacy, and Gothic 3, which I am giving another chance in the wake of the recently released massiver community patch). Accodring to the devs, who have been posting daily on the official forums, the next major gameplay patch is in final beta testing and should be released shortly, which I would guess to be early next week barring problems. They are planning more gameplay patches and are also apparently planning additions to the UI and more graphical bells and whistles (more weather and particle effects). I would guess the timetable for all this is several months. Since CA has been pretty notorious in the past for releasing only one or two patches  for the TW games before ending support (compare this to Stardock--I don't think Brad Wardell could limit himself to one or two patches if his life depended on it), this ambitious schedule seems to confirm the horribly rushed nature of the E:TW release and CA's perceived (and undoutedly correct) need to regain good will from its customers.

For those interested, the first (of supposedly several) major patches to E:TW was released a few days ago. I haven't really played with it yet, but I've seen reports on the forums that many CTD issues have been fixed. (Of course, some of the gameplay changes are proving controversial.)  BTW, I don't really have an opinion on the sieges. I appreciate the graphics in Total War, but as far more of a TBS 4X fan than a RTS fan, after admiring the graphics a few times, I generally autoresolve battles.

I have actualy witnessed a naval invasion albeit a rather pathetic one

While I agree that the Seige AI is terrible and seiges are very unrealistic.  The actual game mechanic makes it very easy to defend, especially against a computer.  If there is an opening in your fort wall or a door that is open, the computer automatically goes there and you can concentrate fire at that point.  Blow a hole in your fort wall and watch the comp line up to be slaughtered.

A person will attempt to be more realistic, so what you can do is concentrate all your forces on one side of the fort and destroy the other building so they dont' have cover.  If they go to the side where all your forces are they are screwed.  If they go the other way you most likely have a stalemate but you have the buildings on your side.

I really wish they would fix the Seige AI though, it really detracts from the game.  I don't like that the troops finally move to defensive positions when the enemy is already climbing up the wall.  You should be able to click on the cannons or something to put them in defensive mode during deployment!

/rant off