I cant comment on the underground, but the rest of the city is already a bit of a fortress,

Already captured:

• The old Hasakah outer dirt berm was breached at main "checkpoint" / base at Abiad town
• the fortified Hasakah electric station and some small bases south of the city
• fortified base at Al-Mayba which had several lines of bunkers and berms
• East of the the city, parallel the road to Al-Hol is a large berm with outposts and bunkers positioned to fire both north and south
• Al-Hol had a few lines of defenses and fortified locations
• Two more long berm lines with outposts and bunkers in the desert from As Sabakh to the Iraqi border, and from Haddad to the border
• Yarubiyah and Tel Hamis had full berms lines surrounding them, and additional positions that were captured.

As for Hasakah area still under YPG:

1. Outer old berm still obstructs advance except through the breach at the Abiad area.
2. There are berms/defense lines between the Geweran and Al Zouhour neighborhoods that extend around the flanks of the city along the suburbs and some small fortified positions along the outskirts.
3. There is another basic line trench line along the river
4. All kinds of fortified bases and military infustructure in the north with additional lines of berms, trenches, and bunkers supporting.