Your frustration is one of the reasons I choose not to play custom scenarios.
It sounds like the author of this scenario designed it just to see how long people could survive before they finally get overwhelmed....sort of a "Custer's Last Stand" scenario.
In your case I would probably just end up fortifying the entire land mass between the river fork with walls and towers galore.
If there are no barbarian catapults, you can build a defensive structure closer to "your side" of the river. If they have them, then you'll need to keep your structures back and away from the river to keep them out of range.
You can also delete the bridges as the warnings of attack arrive. Between attacks I would spend time building some warehouses to store marble, timber, and granaries to store as much food so you have them while the attack rages on.
Deleting the bridges will keep the enemy from across the river from playing a part in the attack, but it will take them a long time to eventually just give up and go away. During an attack all trade usually stops, so a long "siege" will have an impact on your city, unless you manage to build up stores of items and food between attacks.
On that link I provided you with above, the one for the pictures of Nicomedia, there are save files at the very top of the page. Put checkmarks in the box on each one and a download link will appear. They are all in c4s format and ready to place into your Caesar IV Saved Games folder.
Try downloading those saves and let me know if you get them working, and I will then try to find a place where I can upload more for you.
I have pictures and save files from several campaigns for you to look at if you care to, but I don't know where I can upload them for you. My Microsoft Live Space site is near maximum capacity.
My game is an original DVD hard copy and on Windows 8 64-bit you will find that the game installs to the following path:
C:\Program Files (x86)\Sierra\Caesar IV\
Note the "Program Files (x86)", as it is where all 32-bit programs get installed on a 64-bit system.