Only Jim and Juliet have actually professed an opinion about our setting, Pearion! 

I guess it's important to detail a few things about these different settings~

[b]Constantinople: Year 1453[/b]. 
Aggressor: Seljuk Turks (Ottoman Empire) - specifically Sultan Mehmed IIl. Siege was ~53 days long, they had canons, bombs, and a navy during the assault. Constantinople itself was in severe decline. The black plague had swept through the city 100 years before, in 1204 a crusade saw the city sacked and conquered (control shifted to the Latin Empire). Population plummeted from ~400,000 to about ~35,000 by the time Nicaean Emperor Michael VIII Palaeologus recaptured the city half a century later in 1261. 
The city's population was only ~70,000 in 1453 when Memed III conquered it for the Ottoman Empire. About 10,000 of them were soldiers.
Mehmed on the other hand had 80-120 thousand soldiers
The city was a shadow of its former glory when it was conquered! 

We could play a few hundred years earlier during the Abbasid Empire's siege but they never actually captured the city, which would pretty much just mean we were stuck in Constantinople with no way to get out until the siege lifted. Which I mean, we could do~ but it's looking like we're going with a "flee the city" type thing?

[b]Jerusalem: Year 1099[/b].
Aggressor: A rather eclectic group. I'll need to do more research about their demographics, but basically, the European crusaders. Jerusalem had been in Muslim hands since roughly 600 AD. The attacking army is estimated to have been only around 13,000 men, and their low supplied forced them to attack the city rather than wait out an extended siege. The defending army was only ~3,000 strong. At the time Jerusalem was in Egyptian hands, as it had been captured from the Seljuk Turks one year earlier in preparation for the coming invasion... for some reason. 

After the conquest of Jerusalem, the "[i]Franj[/i]" (literally 'Frankish', technically 'European') invaders enacted a brutal slaughter on the populace. Soon after they defeated an Egyptian relief army.
If you guys are wanting to recapture the city, as was suggested earlier, that would actually provide a good opportunity to do it. There's already an army on the march, after all! 

Uhm... other details and stuff. Relatively lazy research given the hour and that I don't particularly feel the urge to research two totally different time periods, only to wind up using one.