Interpretation and effects of fire in built-up areas in Napoleon’s Battles IV.
The fighting in Built-Up Areas (BUA) has been heavily changed.
First, it should be clear that the footprint of an average BUA is simulating a 300 yards x 300 yards area. Due to the scale of the game, this area is not supposed to be fully covered with buildings, orchards, walls, etc. but it does also contain gaps inside. The defensive bonus is simulating the fraction of the area covered with constructions and their type, the bigger bonus corresponding to more and stronger buildings.
Second, almost in every historical account about fighting in BUAs, there are references about individual buildings setting fire due to bombardment or hand to hand combat. Hence, strictly speaking almost every BUA should be set up in fire in NB. However, taking into account the scale of the game, simulating individual burning buildings is far from the scope of the rules. It would be a nightmare for the troops close to (or inside) that building but not for the whole brigade. In NB terms, setting fire to a BUA means that a significant portion of the BUA is burning, and this was not as common as individual burning buildings, hence the low chance for this to happen (in any edition).
It is important to keep both ideas in mind in order to understand what the rule is simulating. Setting fire a BUA means that a significant part (not isolated buildings neither the whole area) is burning. That is why now, with the new rule, it is not mandatory to evacuate the BUA. The rule is assuming that although there is considerable part of the BUA burning there are still gaps and buildings where the units can hold or pass through. Of course this would be really complicated for the troops. The morale would be low and soldiers would tend to fly from the BUA. For this reason any unit in contact with the BUA it is immediately disordered. Remember this would lead to a poor behavior and probably mandatory withdrawals after a round of combat.
In short, the rule is assuming that not the whole area is burning, thus units are not banned to stay there, but players that place their units in a burning BUA will be heavily punished for it. The difference is that now placing units in burning BUAs is allowed although not recommended, whereas in previous editions it was directly banned. It has been considered that there are only spare samples of BUAs burning so heavily as to make impossible to pass through the area, or at least throught the gaps (remember the 300 yards x 300 yards footprint), based on many records about fighting in burning villages.
As an example, when playing the Wagram scenario, plenty of heavy guns and BUAs, at the end of the game most of the BUAs were burning (the chance increases with time) making them impassable if the old rule is used. However, historically, most of the BUAs were certainly burning, but they were still occupied and heavy fighting were developed there, with the BUAs changing hands many times. The new rules about BUAs (fighting, firing, setting fire) simulates what is most likely to happen. Remember that a BUA suffering long term bombardment will probably have his defensive bonus reduced to 0: thus hand to hand combat is likely not to be so bloody; assaults will find it easy to prevail; successful assaults will be, in turn, easily counterattacked by fresh reserves; and if more than this, the BUA is burning, everyone is going to be permanently disordered and probably no one is going to be deployed any more (making the BUA even more untenable).
The chance to set fire to a BUA is low but long term bombardment increases the probability , as an example a 12Fr# firing at a BUA between 4 and 5 inches away has a 30% to score a hit in one round, a 20% to set fire, and the defender has a 50% to stop the fire. These probabilities lead to a 3% to definitively set fire to the BUA (not a big deal), but the probability increases every round as P(n)=1-0,97n being n the number of times the battery fires. Thus the probability is nearly 25% after 5 turns (10 rounds of firing).
