You forgot that if the tower is attacked The Speaker would call every single guardian to defend the tower, and the guardians are the elite fighting force. I'm sure there are police in the city, as well as a basic military with would be used to support the guardians. That would increase the friendly numbers from 1.5 million to maybe 5 million if civilians don't fight too. I would assume that most would help. Increasing our numbers to roughly 7 million. Using your math that would leave 4,000 hostiles per 1 friendly. Assuming that a civilian could only kill two dregs or their equivalent before dying that would change the numbers to 27,998,000,000 H to 5,000,000 F. Now a cop would be able to eliminate maybe 10 dregs, and a Vandal or the equivalents. 400,000 'cops/soldiers' dead 4,400,000 hostiles down. Now we're left with 27,993,600,000 H to 4,600,000 Guardians. Roughly. Now using what your logic the 1.5 million guardians would face the blunt force of the enemy attack. Personally I was able to kill about 5,000 enemies, I would assume that a well trained Guardian, maybe a vanguard could take out 7,500 if properly equipped. That would mean we could kill 11.25 billion agents of darkness. Leaving them with 16.743 billion troops to fight off our incoming 3.1 million guardians. At this point each guardian would only need to kill 5,401 Darkspawns to win the battle. One would have to kill 5,402. It would be a very close battle, that would decimate everyone, but I'm sure we'll come out on top after hours, maybe days of battle. If all guardians become vanguards though we would decimate the darkness.