Of course no fortification can withstand overwhelming force indefinitely, but el-Fasher held out 1.5 years while completely surrounded, which isn't too shabby. (Here's a map from a year prior: https://upload.wikimedia.org/wikipedia/commons/archive/5/52/... It's the small pink blob of army-controlled territory labeled "Al-Fashir" within the gray mass of the RSF.) And the RSF are a formerly government-affiliated civil war faction with a lot more firepower than jihadist militias like JNIM or ISSP.
If some trenches and an earth wall turn a short raid into a long siege, that at least gives the army some time to send reinforcements and attack the besiegers.
They give you time though. It's certainly not perfect, but no wall ever was. You could scale the old wall with a ladder if you wanted to, but it slowed you down and that gave the defenders time to do something about that.
Alone, no. But the fact that modern militaries still build them around bases in insecure areas should give you a moment's pause before dismissing them entirely.
Besieging a city is difficult and expensive. And it fixes a significant attacking army that has to monitor the whole city’s surroundings. They do fall every now and then, but the whole point is that it takes a disproportionate amount of power to do it.
Please do not use German-based transliteration on HN. It is foreign to most readers. Both of the Arabic terms you use have established English transliterations.
If some trenches and an earth wall turn a short raid into a long siege, that at least gives the army some time to send reinforcements and attack the besiegers.