To the Germans' credit, they offered not to take the monastery as a position if the Allies wouldn't bomb it. They only took the advantageous positions after it'd been reduced to ruble.
The Germans also helped evacuate as much of the stuff inside as possible, moving it to the Vatican, where the monks trusted the Germans wouldn't steal the stuff. To further trust amongst the locals and monks, a monk accompanied every truck to the Vatican. As a result most of the library was saved.
Monte Cassino is one of the (many) examples of Allied war crimes and one of the (very) few examples of German honor in WWII.
But given your name, you might see things differently.
The reason almost any above ground strongholds still hold up today is that they're packed with civilians, not because their walls are that strong.
It binds ressources. Lots of them. You even write "slowly". Meaning fortification is still effective.
While the enemy artillery is busy bombing stone, your troops can attack other targets from from other angles.
It didn't matter, the Monte Cassino was a meat grinder for the Allies.
Anyway, why dredge WWII era stuff? The US got spanked in Afghanistan by an irregular militia with bolt action rifles. And their tactic is simple as it is brilliant - negate the advantage of air raids by fighting as close to the enemy as possible.
The same tactic was used to good effect, by the Viet Cong.
https://www.google.com/maps/place/Nad-e-+Ali,+Afghanistan/@3...
"On 15 February American bombers dropped 1,400 tons of high explosives, creating widespread damage.[6] The raid failed to achieve its objective, as German paratroopers then occupied the rubble and established excellent defensive positions amid the ruins. "