Well, I've read the passage you speak of and it seems to be pretty straight forward to me: Caesar was besieging the Oppidum and found an oppertunity to attack and was forced to retreat.
These were the major events:
Prelude: Caesar was about to retreat from his position at Gergovia due to the fear of being cut of from the rest of his troops which he wanted to merge with and thus gain the strength to face Vercingetorixs rebellion.
1: An opportunity appears as Caesar comes to know that an enemy (Gauls) position is especially vulnerable.
2 (midnight): He sends some cavalry and decoy troops to give the enemy the impression that the position is threatened. A legion follows, but hides along the way (presumably to make the enemy (Gauls) even more insecure).
3 (Night/early morning?): The Gauls send all of their troops to the vulnerable position, leaving their camp empty.
4 (still dark? Dusk?): Caesar in a stealthy manner sends (infantry) troops to the now empty Gaulish camp. They were not to be observed.
5: The infantry from point 4 is sent (marching up 1200 roman double steps along the hill) in along with the Aedui to another hill to the right.
6: The Romans from point 4 encounters a wall half way up and quickly overcome it.
7: The three Gaulish camps (from point 3) are captured. Caesar calls a retreat but only the 10th responds. The other legions can't hear the signals.
8: The troops that did not hear the signal to retreat reach the city walls. Some legionaries scale them.
9 (reported to be late into the battle. Turning point): The Gauls from point 3 hear the battle cries and return. The Romans find it difficult to hold against the fresh Gauls and are pushed back from walls and gate. Many Romans are killed.
10: Caesar order several cohorts of the 13th (480 men each) under T. Sextius to advance and take position and thus make sure that the enemy (Gauls) did not pursue the men retreating from the walls too far.
11: Roman at the walls retreat. Caesar moves his own legion a little bit forward to stand ready.
12: Though fighting.
13: The Allied Aedui appear on the right side. The fighting Romans think them to be enemies and are thus terrified. Heroic action at the middle wall (?).
14: Romans in full retreat. The 10th legion (point 7) and the cohorts of the 13th (point 10) under T. Sextius supports their comrades and prevents to a certain extent the pursuing Gauls.
15 (end): The legions reach flat ground and make a stand.
16 (end): Vercingetorix orders the Gauls to retreat and they comply.
Caesar also notes that almost 700 men were killed during the action, among them 46 centurions. The next day he gives a speech chastising the troops for their failure to comply with his orders, but he also encourages them (as he is afraid that moral will be lost otherwise). The troops marches to the field for another two days, ready for a battle, but the Gauls stay inside their walls. Caesar therefore retreats to the land of the Allied Aedui.