Well 60,000 would be easily routed. If you got to 90,000 then maybe it would be a real fight. But men behind high walls that can quite easily man them. The walls are 100 feet tall for the first level alone and 700 feet from bottom to top. Looking at all the depictions I can find the city does not seem much wider than it is tall. With circular walls this leads me to believe that the length of the wall can not be much greater than 2400 feet. That means that they can have roughly 12 men per foot of exterior wall. So basically they can afford to man the lowest wall shoulder to shoulder with a lot of men left over. Even if all the walls were the same length they could STILL man every wall shoulder to shoulder with 3 out of 10 troopers left over. With that kind of fortification and concentration of firepower no conventional force would be able to deal with them. Add in artillery from elevation and no trebuchets would last long enough to do more than niggling damage. The only hope that the attackers have is that the defenders moral will completely snap.
However 30,000 men is far more than needed to defend. in defense of this I point out that the reinforcements including prince imrahil only were around 2700 men. I haven't found any numbers for how many were actually there but I'm guessing that 30,000 men is more than the defenders had cannonically.