Assaults can be a powerful tool in warfare, speeding up the capture of enemy cities. However, assaults are never absolutely necessary, and usually long sieges will be cheaper in men (and thus money). So there are tradeoffs. This article discusses assaults, and when you should use them, as well as discussing useful tricks you can use with assaults.
When to assault
There are no hard and fast rules. However, generally you should not attempts assaults without a significant edge. In a battle, with no edge or even a slight disadvantage, you can always do probes, where you observe the die roll, and if it is bad, flee and try again. But you cannot stop an assault once you start it, so in most circumstances you should not risk assaults without at least parity in DRM, and ideally several DRM worth of advantages.
There are just two sources of advantage in assaults:
- superior land tech, which gives higher morale, fire and shockpower, and also possibly CRT advantages
- leaders, including the effects of the Offensive and Quality domestic policies.
For the defender, the wall is counted as an additional DRM, so long as it is not breached. A level N fort gives a +N DRM. For example, a level 2 fort gives a +2 DRM for defenders.
A rule of thumb for assaults is to count significantly higher morale as like a +1 DRM. And count each CRT table advantage as worth +1 DRM. Add another +2 DRM for the side with a fire phase if the opposition has none. Add these to the DRMs from your leaders, then subtract the DRM that the default enemy has, and also, the fortress level (if unbreached).
-1: don't bother 0: assault possible, but likely to fail +1: risky +2: good odds of success, at least 50/50 +3: odds are maybe 75% +4: sure thing
So, in general I don't do assaults against equal-tech enemies, unless I've gotten a breach or a strong leader. Having a good fire bonus is the most important thing, since the fire phase happens first.
But hey, if you have some small amount of infantry controlling a siege among a dogpile of AI countries, then an assault can make perfect sense. You stand to lose little and gain much. See the article on stealing sieges.
Even when an initial assault fails, the damage done to the numbers of defenders of the fort will still remain. Also, the morale of the defenders does not recharge until the end of the month; and the morale of defenders will only recharge if there is no additional assault at the time.
It is possible to continually assault a fortress so that the morale of the defenders is consistently lowered without ever having a chance to be recharged. Troops for additional assaults may either be in motion in the province where the assault occurs or may be destined to arrive before the following month. Troops destined to arrive should be given a movement path that does not end in the assault province, since that could cause the troops to add to the current assault if not carefully managed.
After a failed assault, it is possible to click "cover" to break off a small amount of troops to keep the assault progress. Send the remaining broken morale troops in motion out of the province where hopefully they may be merged with other troops to more quickly charge their morale. If troops are in motion in the assault province, stop the full morale troops and put them on the siege. The weighted average of morale should be enough to start another assault; but you should put the low morale cover force in motion out of the province before starting another assault. The method is the same for troops destined to arrive before the end of the month except you may have to wait for the troops to arrive before starting the next assault. Fresh troops must be available for each month of the ongoing assaults.
Note that the weighted average of morale needed to start the assault is only computed when a new band of troops is stopped to be placed on the siege. Also, the morale is recomputed at the time an assault begins; the morale may change since troops on the siege may have been placed in motion.
Also note that it is only necessary to have begun a siege, but it is not necessary to actually have an entire siege force in the assault. Technically, an assault may be started with only enough men to cover the fortress; the other men needed to start the siege can be placed in motion. A cover force is so small a force that it is not likely to succeed and will not preserve the assault progress should it fail. However, using a small force to begin an assault and then stopping men that are in motion can help keep morale higher.
Assaults may be most useful in provinces where there is low supply. In low supply provinces, you would not want to keep a large number of troops in the assault province as is the case for having troops in motion within the province at all times. Also, you would like your assaults timed so that your troops dwindle somewhat by the end of the month, and each assault ends early in the month so that the low morale troops can get out before the end of the month. Beginning each assault on the 20th of the month or so may be best; but it depends on your troops' morale level. Timing fresh troops to arrive by the 20th of the month is therefore recommended. Another force of fresh troops should also be scheduled to arrive the last day of the month. But those troops are a last precaution; their movement should be cancelled on the 29th unless the assault is going poorly and either has failed or may fail before the end of the month. If the assault only has a chance of failing before the end of the month, the advancing force can be split to give only a small boost to morale in order to keep the assault going to the next month.