MOSUL, Iraq — In the last square mile of Islamic State territory in this city, terrified families trapped in their basements are bracing for a final ferocious showdown.

As many as 150,000 residents are crammed into Mosul’s Old City, as Islamic State fighters fortify their positions in the warren of narrow streets and alleyways [....]

The punishing eight-month battle for the city has taken place in heavily populated neighborhoods, leaving entire streets in ruins [....]

In the middle of the historic city center lies the Great Mosque of al-Nuri [...] where Islamic State leader Abu Bakr al-Baghdadi declared a caliphate three years ago. The fighters have hunkered down in surrounding alleys — nine or 10 in each, according to another resident who recently fled — and zip around on motorcycles.

Lt. Gen. Stephen Townsend, commander of the U.S.-led coalition backing Iraq’s fight with airstrikes and military advisers, recently described Mosul as the toughest urban warfare he has witnessed, or even read about, during his 34 years of service. But Iraqi and U.S. military officials expect the final push into the Old City to be more brutal still as the militants make a last stand.

Massacres of civilians attempting to leave have deterred many from trying. In the worst incident, more than 100 people were gunned down near the city’s Pepsi factory this month. The militants also confiscate identity cards; some men are scared to leave without them because security forces might suspect they are Islamic State fighters [....]