Actually, I think I understand what happened now. The garrison on turn 19 wasn't the army that I'd ordered into the city on turn 18. I'm guessing that army was caught outside the city and destroyed, which would explain why the invading army was so weakened. I must have been careless about autoresolv...

I started my first game as the Iberii and on turn 19 the Lusitani sent an army to besiege my city of Corduba. Within the city were 8 garrison units: 4 medium infantry and 4 skirmishers all of which were at full health and effectiveness. The siege rating was -2 in favor of the defenders. I had 1 wall...