The army consisted of thirty thousand men.

If that army could be brought to Alexandria and kept under Pothinus's orders, Caesar and his three thousand Roman soldiers would be, they thought, wholly at their mercy.
There was, however, one danger to be guarded against in ordering the army to march toward the capital, and that was, that Ptolemy, while under Caesar's influence, might open communication with the officers, and so obtain command of its movements, and thwart all the conspirators' designs.

To prevent this, it was arranged between Pothinus and Achillas that the latter should make his escape from Alexandria, proceed immediately to the camp at Pelusium, resume the command of the troops there, and conduct them himself to the capital; and that in all these operations, and also subsequently on his arrival, he should obey no orders unless they came to him through Pothinus himself.
Although sentinels and guards were probably stationed at the gates and avenues leading from the city Achillas contrived to effect his escape and to join the army.

He placed himself at the head of the forces, and commenced his march toward the capital.

Pothinus remained all the time within the city as a spy, pretending to acquiesce in Caesar's decision, and to be on friendly terms with him, but really plotting for his overthrow, and obtaining all the information which his position enabled him to command, in order that he might co-operate with the army and Achillas when they should arrive.
All these things were done with the utmost secrecy, and so cunning and adroit were the conspirators in forming and executing their plots, that Caesar seems to have had no knowledge of the measures which his enemies were taking, until he suddenly heard that the main body of Ptolemy's army was approaching the city, at least twenty thousand strong.