With the exception that there was never any "guild of assassins", it sounds like Erikson may have adopted something he read about Classical Greece. Since most Greek city-states lacked the money and manpower to commit to prolonged sieges or build elaborate siege machines, the most common way to seize a city was either to attack by surprise (the end of the Trojan War being the archetypal example) or to encourage treachery from within. Fifth columns and treason were such a common feature of siege warfare that Aineias the Tactician's treatise On Siege Warfare (4th century BC) is predominantly about how to expose conspiracies, foil plots, and forestall common tricks and devices used by traitors to give the city over to the enemy.

Generally speaking, the people who turned traitor were factions or individuals disaffected with the current regime of a given city-state. Such people were almost always available. In democracies, oligarchic factions may be plotting to seize power; in oligarchies, democrats may be conspiring to overthrow the regime. In both systems of government, factions unsatisfied with their own level of influence might be tempted to rebel and adjust the balance. Some hoped to gain power or the favour of foreign overlords; others hoped to be absolved of crimes or for others to be restored from exile; others simply hoped to get rich. The Battle of Marathon was fought to prevent the Persians restoring a deposed Athenian tyrant who still counted on the support of his old friends inside the city. During the Peloponnesian War, nearly every Greek state that was part of the Athenian Empire had some pro-oligarchic elements courting Spartan support to start an uprising. Simultaneously, nearly every state in the Peloponnesian League nurtured some pro-democratic faction hoping the Athenians would come and back their rebellion.

As a result of this ubiquitous discord, all an invading power had to do was find a way to contact the disaffected factions in a given city-state and promise them that they would be co-opted in the regime they meant to set up. This was not particularly difficult, because the prospective fifth column would often take the initiative:

[The Spartans] at once marched against Torone in Chalkidike, which was held by an Athenian garrison, having been invited by a few persons who were prepared to hand over the town.

-- Thucydides 4.110.1

[Dionysios of Syracuse] led his army to Katane, for Arkesilaos, the general of the Katanians, had offered to betray the city to him.

-- Diodoros 14.15.1

Two men, Pasimelos and Alkimenes, undertook by wading through a torrent to effect a meeting with Praxitas, the Lakedaimonian polemarch, who chanced to be on garrison duty with his unit at Sikyon, and told him that they could give him entrance to the walls which reached down to Lechaion.

-- Xenophon, Hellenika 4.4.7

Now, regime change in Greek city-states was rarely a bloodless affair; the few occasions in which a defeated faction was simply incorporated into the citizen body and involved in the new system of government were rare and widely celebrated. It was more common for the switch to a new system to be accompanied by executions, mass exile, and often wholesale massacre. Oligarchic factions in particular were rarely trusted with their lives, since they had an annoying tendency to gather mercenaries while in exile and seek to reassert themselves by force.

Combine this with the atrocious excesses of sacking a city and you can imagine the horrific scene that would follow when a city was betrayed to the enemy. The typical result of any successful siege was an explosion of violence:

Prokles, the commander of the Naxians, on being won over by great promises, delivered over his native city to Dionysios, who, after paying the promised gifts to the traitor and granting him his kinsmen, sold the inhabitants into slavery, turned their property over to the soldiers to plunder, and razed the walls and the dwellings.

-- Diodoros 14.15.2

When a city had been given over by treason, that violence was specifically targeted at whatever faction or group they were hoping to depose. Specific families or classes of the citizen body were wiped out; their possessions seized; their wives and children sold into slavery. For the invading force, it was useful to encourage such essentially internal violence, because the more leeway the fifth column got to eradicate its opponents, the more gratitude and loyalty they were likely to show to their liberators, and the more stable their puppet regime was likely to be. Sometimes the violence could continue well into the reign of the new regime, as happened at Athens after the Spartans won the Peloponnesian War.

The rich were a common target for several reasons. First, to put it crudely, they had more stuff worth taking; driving them into exile or murdering them was a great way to loosen up some cash to pay off the supportive invader or prop up the new regime. Second, related to the first, they had the means to make themselves a nuisance to the men newly in charge - be it the connections to potential counter-revolutionaries or the money to raise a private army against the new government. Third, they were more likely to be members of the ruling class and entertain ambitions to power. It seems to have been fairly well-known who was in which camp when civil war broke out in a Greek city-state; to prevent further upheaval, it was easiest simply to tear out all potential rivals from the citizen body. This was the only way in which a new government system could be secured.