The Napoleonic Code of Conduct and the Official Honors of War Guide for War and Peace 

A recent game has shown that both the rules and these errata as published earlier are not completely in alignment, specifically on the Assault conditions. Hence my humble attempt to help clarify is added in blue italic
Note that the following statements remain ultimately true and valid.

However, the rules are written for the gamer with a higher tolerance of ambiguity.  To those of us who prefer airtight rules, with cut and dry regulations (the kind that Napoleon himself would be proud of), War & Peace can become frustrating.  Therefore, the following is an attempt to cleanup the rulebook and make this excellent game enjoyable to all players, even the rules lawyers. 
The format of the following errata is to identify the rule in question first (e.g. "C.2" means rule book section "C. Mapboard", paragraph #2.  "The mapboard is composed of four..."), followed by a clarification, modification, or change, written with the intent of tying up loose ends, closing loop holes, eliminating ambiguities, or otherwise making the game "play Better".  In some cases, where explanation is needed, a brief discussion is provided to explain the reasons behind the rulings. 

Note that these are the author’s interpretations and not the rulings as such.

The official rules contain two sections on page 10, first column.

S.8. Note: In order to assault a city, the active player must initiate combat in the hex at the start of the Combat Phase and he may be required to fight a field battle in the hex before the city may be assaulted. A player is permitted to resolve a field battle and afterward assault a city in the same hex during the same Combat Phase. During each round of combat, the assault may be continued or broken off" at the attacking player's option, exactly like a field battle.
This rule unambiguously states that one must have moved into the hex prior to this combat phase, either during the movement phase or during a previous turn.

S.9. At the start of any round of combat, if all enemy strength points in a hex are inside the city, the attacking player may immediately advance any or all of his attacking units into the hex to besiege the city.
This rule states only that one can move into the city hex during the combat phase to start a siege, it does not mention anything about an assault.

Summary: 

The rules state, if you are in the hex at the BEGINNING of the combat phase, you can assault, if not, you can not in this turn.

Details:
L.5. Addition: Units overrunning enemy units in a city hex (units outside the city), may elect to stop their movement on that city hex, thus trapping any opposing units inside the city.  Also, if all of the inactive units are declared inside the city, the moving force may enter the hex (at no additional overrun costs), and may leave or stay.  In cases where the overrunning force stays in the city hex, the Combat Phase starts the same as if the moving force was adjacent, initiated an attack, the defenders all withdrew into the city, and then the active units entered the city hex (to assault or besiege).  
	This ruling is only different from the original overrun procedure in so far as it allows the unit to enter a city hex at 4:1 ratio minimum (being defined as “overrunning force”) at no additional MP cost, so only the terrain cost is needed, (minimum 1 for clear). Note this can happen during the movement phase as the overrunning force is at the correct ratio. Note this is not a clarification but an addition to the overrun rules.

S.5. Clarification: Units inside the city may only be attacked via the process described in the city assault section, and only by units in the same hex.  Enemy units may not enter the city hex until there are no friendly units outside the city, except for overruns.  If a subsequent field battle occurs in that hex, and other forces of the side that is "outside" the city wish to commit to that field battle, they must do so from an adjacent hex - they do not enter the hex, (note; they may enter the hex after the field battle if the conditions for a city assault are fulfilled and a city assault is executed).
	This ruling is consistent with the original rules as to confirm that one needs to be on the city hex before one can initiate an assault. It stipulates that one needs to fulfill the conditions for a city assault, conditions which clearly state that (see official rules) one must be in the hex at the START of the combat phase.

S.6.b. Clarification: This rule also only pertains to situations where the combat round starts with both sides not occupying the same city hex.  Both sides can never be both outside the city at the start or end of a Combat Phase.  	
Explanation/discussion:
	The important thing to remember with the above two rulings is that it is generally not possible to engage in a field battle with units in the same hex.  The usual course of events is that the active player will move adjacent to a city hex occupied by inactive units, and announce his intentions to attack that hex.  The inactive player must declare which of his units are inside or outside the city.  If he declares that there are any units outside the city then a regular field battle is fought.  Once there are no inactive units outside the city at the start of a combat round, the active player may move into the hex, and either assault the city or initiate a siege. 
	This bold text just above is the only statement in the errata which is in clear contradiction with the other clarifications and with the original rules. 

S.8 Clarification: Units that assault the city, must first move into the city hex, (i.e. assaults against the city are not made from an adjacent hex).  Field Battles may be fought either from adjacent hexes or within the same hex, but may only be fought within the same hex if all units involved in the initial combat round started the Combat Phase in that hex.  
This ruling again is consistent with the original rules as to confirm that one needs to be on the city hex before one can initiate an assault.

Additional comment/clarification:
During the debate reference was made to the fact that the rules say that an assault is just like a field battle, which means ADJACENT not on. 

This is not completely accurate. What the rules actually state is:

S. 8. The procedure used to resolve an assault is identical to that used to resolve a field battle except:
a. The strength of the units inside the city is doubled when determining the combat odds ratio.
b. Units, which possess a basic morale value of zero, are considered to possess a morale value of one when inside a city.
c. The dice roll used to resolve the combat is not modified for terrain effects or Entrenchment.
Note: In order to assault a city, the active player must initiate combat in the hex at the start of the Combat Phase and he may be required to fight a field battle in the hex before the city may be assaulted. A player is permitted to resolve a field battle and afterward assault a city in the same hex during the same Combat Phase. During each round of combat, the assault may be continued or "broken off" at the attacking player's option, exactly like a field battle. 

This automatically implies that one has to fulfill the assault conditions before one can apply the assault procedure.

My suggestion remains to stick as close as possible to the original rules.
Summary: 
The rules state, if you are in the hex at the BEGINNING of the combat phase, you can assault, if not, you can not in that turn.
Respectfully,
The Emperor Napoleon