In his defense, I don't think they were that unprepared because IIRC they took Torval before the Summit was over, so Ta'fel seemed more interested in just seizing the south. The South is more of an NPC nation than a PC nation, and the redguards weren't given a proper chance to defend despite being a PC nation, and in a collab post that Pyandonea still needs to stick up the imperial navy did deliver supplies and seige equipment to Torval once Ta'fel had it (And according to the next piece of the collab, it was 1.5 days earlier than the current time that they did that, though that means the Argonians have likely arrived by now, though he could have started marching then and gotten to Senchal a few days later) Though writing 'a few hours later' and having marched an army from Torval to Senchal is pretty insane shit. Days would be more the measurement, I think, and by then the argonians will likely have arrived, unless they have encountered pirates