Wa Alaikum As'salam,
The people had an army and weapons. It's just that that area was a weakness. So Dhul-Qarnayn, peace be upon him, sealed it. So coming around the barrier was not possible for the Gog and Magog enemy. It would've probably been too costly and risky for them. And the enemy was most likely a tribe from Gog and Magog, and not all of them. Probably they were raiders and thieves. Sort of like Barbarians. So Dhul-Qarnayn had put the barrier that finally stopped them from pillaging and stealing.
I hope this helps, insha'Allah.
Take care,
Osama Abdallah