Ah now I see that there is a problem with my method because apparently the trigger_once does not reset every round so it will hurt the players only the first round (a very unwelcome handicap in the cs code apparently
And since the trigger_once doesn't have an on/off state you can't reset it with trigger_relay each round.
So that makes the map a little tougher to make (now I see why the original one uses teleports).
Now I see two possible ways of making this map:
One includes a complicated combination of a multisource, trigger_multiples, trigger_relays and a func_door (no, I'm not joking, you need an inaccessible func_door - with it and a trigger_relay you can reset an entity's state at the beginning of a round). I would not suggest using this one because it gets too complicated fast so I won't even explain it.
The other is making separate rooms (inaccessible) with trigger_multiples and trigger_teleports. This is how it's probably done in the original map and this is what I'll use in the example map.
EDIT: all this teleporting is tougher than I thought. You have to make a trigger_teleport under every single spawn point and I'm getting a lot of worldspawn deaths when I play with a lot of bots for no apparent reason.
...and that's the bottom line because Mate de Vita said so.