Sigil can fall, if you remove the Lady from the equation.
1.But what I contend is due to the fact that only the Lady is able to alter Sigils portals, they would be unsealable in her abscene. Given that fact, no occupying force would be able to hold the city proper. Attacks can come from all sides, and even if you guard the large and well known portals, it would be impossible for Demons and Devils to hold the city.
There would be about 2-3 weeks were the demons and devils would have street to street fighting, and despite their number, Sigil residence would bleed them. By the time they had cleared the city, (no small task given what is running around in it, and under it), they would still be suffering from Guerilla style attacks from the many open portals. You would literally have to kill every non-demon and devil off in the city and then pour the city so full of them that they could guard every tunnel, portal and gate way. Your offense would stall just to hold the city, and unless you took every gate town as well, you wouldn't stand a chance of even beginning to hold.
Sigil wouldn't turn into a Demonic/Devil stronghold, but rather the equivalent of Vietnam or Seriavo, only 666 times worse. The overlords would keep pouring troups in and Sigil would keep eating them. You wouldn't even need a huge number of troops, just a couple high level adventuring parties would suffice.
And given the number of people who would want Sigil back, they would be depleting their forces rather quickly.
And even if you could seal portals, they just pop up everywhere here and offer much the same problem. Just something to think about.