The impregnable regime bases are in the Qassioum, which, in addition to being Assad's domicile, is also the headquarters of the Republican Guard, al-Drejj (to the north of Qudssaya), Mazzeh, home a military airport and army bases belonging to the Fourth Armored Division, Qataneh, and Kiswah.
The rebels face two major obstacles in any concerted foray into central Damascus. First, there is a greater concentration of conventional regime forces here than anywhere else in Syria, including not just the bulk of the two praetorian divisions but other divisions and the now mainly Alawite-staffed Special Forces. Rebels have never encountered this quotient of Syrian regulars before because these troops constitute a loyalist hardcore, the possibility of prompting defections is much slimmer. This is to say nothing of the capital's buildup of the Popular Committees and Jaysh al-Sha'bi militias which stand to take the place of Syrian army in a coming sectarian free-for-all. Urban combat here will be longer and bloodier than it was in Aleppo. Imagine every street another Salaheddine.
The second problem is topographical. All of the regime installations cited above are in elevated positions meaning that, as rebels advance into downtown Damascus, a steady barrage of rockets and artillery can rain down on the capital until there aren't any buildings left standing. As grim as it may sound, this may actually constitute the rebels' end-game in the absence of foreign intervention.
How does a guerrilla insurgency make up for its lack of firepower or an air component? By using the other side's to do its bidding. Typically what has happened in other fought-over swaths of Syria is that the rebels have laid siege to regime installations with their own rockets and artillery for the purpose of infiltrating them and confiscating whatever materiel they can carry or drive off with. (This ranges from Kalashnikovs to surface-to-air missiles). The regime inevitably responds to the loss of its own strategic terrain, and the prospect of better-equipped enemies, by bombarding these sites and rendering them inoperable even when the rebels are flushed out.
According to analysts I've spoken to, there is simply no way that the rebels can penetrate the Rif Dimashq military installations given their current capability, even with Croatian rocket launchers and recoilless guns. Since they don't have a no-fly zone or close air support, or heavier caliber weapons the West has been reluctant to supply them, they will likely resort to the kind of pinprick measures - suicide and car bombings - we've seen used against NATO and coalition forces in Afghanistan and Iraq.
Rebels will thus act as both moving targets and guidance systems for the regime's own war machine, raiding one base in the hopes that it will be cannibalistically powdered by the adjoining. Jahbat al-Nusra is well-poised to be the vanguard fighting force in Damascus since its militants have the fewest reservations about sacrificing themselves. Idriss' Supreme Military Command, although it disclaims Nusra membership in its ranks, is not above partnering with the caliphate-minded jihadists for precisely this reason.
Exhausting the regime physically and psychologically will proceed apace with these attritional tactics. In addition to casting doubt about the survival of high command figures, the rebels will also try to infiltrate the inner sanctum using proxies (chauffeurs, cooks, aides-de-camp). In February, The New York Times quoted one Syrian Army soldier from a distant province who was assigned to manning a checkpoint near a railway station in Damascus where troops and tanks were being imported: "I didn't see my family for more than a year. I am tired and haven't slept well for a week. I have one wish - to see my family and have a long, long sleep. Then I don't care if I die." It's this sense of defeatism the rebels are counting on.
But be under no illusions about how long it will take or at what price the fall of Sham will come. Since the siege of Aleppo began last summer, 10,000 have been killed in that province, a million have fled, another half million are internally displaced, and out of a total population of 2.4 million, 2.3 million are living in areas needing humanitarian aid. Not for nothing did the rebels name the battle for Damascus the 'Battle of Armageddon.'