I found this tip somewhere in other thread and tried it. It worked so great that I decoded it deserves a separate thread. If your already promoted city raider siege unit has less than 60% chance to withdraw and you have extra siege units in your stack not promoted yet - promote them with barrage to suicide them. You deal less damage to top defender yet more damage to everyone else. This often saves you a few units. If you attack enemy in open (no city raider) while Drill slightly increases the survival chance barrage is usually better, especially if you have enough normal attack units to finish damaged enemies. I'm not sure about when to use drill, probably you should use it if you have too many promotions (like with Cyrus) you can make CR3D4 siege that is great against defenders at least slightly scratched. Still not sure how it compares to CR3B3D1.