You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
alternatehistory.com
IOTL, the armies of the Ummayad Caliphate besieged Nicaea after taking Iconion. This took place about ten years after the second seige of Constantinople.
The Arab armies nearly took the city, and even managed to breach the city's walls.
Now, what would a victory there open up for them? What it opens up is the Western Anatolian plain. Although they took control of many of the cities during their earlier invasions, and used those cities as bases for their fleet, that area wasn't really under there control because they left Byzantine forces behind them. Here however, with control of Nicaea they can prevent the Byzantine forces from being a threat. This would effectively lead to the fall of Asia Minor, and leave the Byzantines with their Balkan enclaves.
For reference, here is a map of Byzantine territory under the reign of Leo III the Isaurian:
Here is Nicaea: