1) We raised the men's morale by lighting the signal fires.
2) Whatever enemy is left would have been pulverised by the catapults that we helped to raise.
3) We secured the Keep Depths and Larder (not too sure about the stinking food though), and might have walled up the breach (whether with stones or demon bodies is anyone's guess though).
4) We saved the walls of the Keep, by destroying the catapults, trebuchet, and the Siegebreaker Assault Beast, hence reducing the demon horde's offensive power.
5) We depowered the demon horde by destroying the Sin Hearts.
6) Finally, we removed whatever semblance of leadership they have (tactical or otherwise), having destroyed Cydaea and Azmodan.
I don't think it takes a tactical genius to recognise that the tides of battle have been turned overwhelming in our favour. And yet, after Adria's betrayal, and the start of Act 4, we seem to have lost some people, who were still alive and celebrating at the end of Act 3 (but before Adria's betrayal).
List of notable dead:
1) Former Mayor Holus
2) Rondal and Marta
Three thoughts came to mind, but I managed to debunk all the theories (somewhat). They are linked to the three possible killing forces still existing around that time:
1) Remnants of Azmodan's army - Devoid of any probably command and control, they might have committed to a last-ditch attempt to breach the walls, dying to the last demon, but managing to break into the stronghold and killing some people there. However, based on what I've mentioned above, the demon army should have been too fully trounced by us to be even capable of that.
2) Adria - Well, she's evil, so I assume she'd kill people. However, she "portal-ed away", and I doubt she'll choose to "portal" down into the stronghold.
3) Diablo - Likewise, he's (the Prime) evil, so I assume it'll kill people too. However, it's shown that it "portal-ed" into Heaven.
I wonder what could have happened...