The Focsani-Namoloasa-Braila(FNB) line was about 180km long and situated on the Siret river bank, beween the impassable Carpathian mountains and the Danube Delta so it couldn't be flanked. It had about 1600 concrete pillboxes, 1800 emplacements for AT guns and MGs, 80 km of AT ditches, 18m wide and 6m in depth in front of the exposed areas and the usual trenches and mine fields.
At the time of king Michaels coup, the german 6th and 8th armys, together with the romanian 3rd and 4th, about 500k strong, were in full retreat and relocation towards the mountains and the FNB line. Besides that there were about 1 mil romanian reserves. The Ploiesti heavy AA guns were also being redeployed there. During the Iasi-Kisinev offensive, the soviet high command did not even plan to pass the FNB line at that particular moment. Sure, if they alocated enough resources and men they could eventualy do it, but the main thrust towards Berlin was through Poland, so occupying Romania wasn't necessary to defeat Germany.
Marshal Antonescu hoped to inflict as much damage to the Soviet Army trying to infiltrate south of the FNB line, that Stalin would have to reconsider, negotiate and offer better truce terms. Not only offer better terms, but also respect them afterwards, because the Soviets only respected power.
The Gustav line denied access to the Allied forces for about 4 months. The casemate density on this line was lower than that of the FNB line, and had a lot less troops.
However, king Michael was tired of living in the shadow of the man who put him back on the Throne. He was convinced by the politicians and military he kept around him of his providential role in the years to come, if he himself would be the one signing the truce with the Allies and not Marshal Antonescu. Having in mind that after the war, king Michael received the order of Victory, we could say that Stalin considered he had a big contribution in defeating Germany and/or helping SU in securing a large part of the Balkans. The rest is History.
http://www.academia.edu/19208142/Enemy_at_the_gates_a_reconsideration_of_the_purpose_and_potential_of_Romania_s_last_defense_line_against_communism_the_fortified_line_Focşani_Nămoloasa_Brăila
Who is curious to know a lot more about the FNB fortified line can acces this link.