There are a few things in this passage that I’m not sure I understand. Seneca is talking about Caesar.
dum ille pontes navibus iungit et viribus imperi ludit, aderat ultimum malorum obsessis quoque, alimentorum egestas ; exitio paene ac fame constitit et, quae famem sequitur, rerum omnium ruina furiosi et externi et infeliciter superbi regis imitatio.
Am I correct in taking obsessis to refer to the Roman people? While they were besieged, there was also present the greatest of evils: a shortage of food. I don’t know what to make of constiti. Is it from consto or consisto? Could it mean “[The situation] consisted of near destruction and famine” or “It cost (the people) near destruction and famine.”? Your ideas would be much appreciated.
Yes, obsessis means ‘those under siege’. It’s in the dative because of aderat.
I take exitio, fame and ruina as ablatives of price, i.e. these were nearly (paene) the price the city paid for his recklessness. Consto used this way just means ‘to cost’ (OLD, sense 11). Constitit agrees with the subject imitatio.