MOSUL -- Iraqi forces backed by the U.S. military are making gains in their push to drive the Islamic State of Iraq and Syria (ISIS) out of Iraq’s second largest city, Mosul.

It’s been more than 100 days since the battle to retake Mosul began. Even with the backup of U.S. airstrikes, Iraqi forces have only recaptured the eastern side.

As we drove through the streets, we witnessed the trail of destruction from the fighting that had raged here. The journey was made more eerie by the fact that ISIS militants and sympathizers might be watching. Iraqi commanders believe a number of ISIS fighters were locals, and simply melted back into the community.

The trail of destruction from fighting that raged in Mosul. CBS Evening News

Iraqi Special Forces took us to Mosul University, the scene of a ferocious battle to recapture an ISIS HQ. Retaking the university was a strategic and symbolic victory for Iraqi forces. It served as an ISIS headquarters -- a command and control center, which made it a prime target for U.S. and coalition airstrikes.

Captain Saif Ali said some ISIS fighters fought to the death.

Iraqis believe ISIS used the university as a missile and mortar factory, its labs to develop chemical weapons. We even found ammunition stashed in a busted freezer. We also saw signs of what it used to be: the lecture halls, a campus book shop, dorms for some of the 30,000 students who used to study here.

Mosul University was the scene of a ferocious battle to recapture an ISIS headquarters. CBS Evening News

Closer to the frontline, soldiers brought us to a home ISIS had fled from, passed a stripped out SUV in the process of being turned into a suicide car bomb. Inside, they showed us a pile of toys, confiscated from children, forbidden under ISIS. A normal childhood put on hold.

The fight has come to a standstill right along the banks of the Tigris.

Major Arkan Hashem, battalion commander with the federal police, said he and his men come under a regular barrage. He showed us where mortars had come in just a few hours before we got there.

Morale is high as Iraqi forces prepare for the final assault against ISIS in Mosul. What no one knows is what ISIS has left -- how many bullets and suicide car bombs -- and whether they’ve saved their strongest fighters for a final stand.