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.

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.

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.

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.