Síndrome da torre

So we sat here for a while trying to figure out what it meant to complete R8 (B) level with only Sentries. We completed the level using Stealth, Sentries, and running out of ammo at the final door with the final Scout, even though it is impossible to do on (2) or (3) even with a lot of resource boosters and it was unclear if we could use anything other than Sentries. Hope this is sufficient.