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.