Sounds good, just want to make some changes and elaborate. Inside the courtyard, between the preliminary and secondary gates, there should be three NULL cannons targeting the attacker's MCC. Defenders can upgrade to have more. If the MCC is destroyed, the defenders win. On second thought, the NULL cannons will be farther into the compound. Defenders will purchase different turret installations and maybe buy added on sniper perches. Maybe there will be some bases that can be attacked from all sides and therefore have more entrances. Anyway, love the idea of a sort of fort siege and look forward to playing it.

Yeah, my idea was a base idea, to be worked on for sure. My first Idea was for the defending team to simply defend, but you ideas would also work.
Maybe once the gates fall(This alone would have it's own time limit), the NULL cannons become active, defending team need t keep them programmed(Hacked as in skirmish). Meaning the attacking team would need to complete their tasks before their ship is destroyed, leaving them with the choice of trying t slow down the NULL attacks, or to procede with main tasks.

The only problem with multiple entrances would be te amount of players, 16 per side, If theres is too much, the battle can become to thin, for either side.

Another Idea I had is Ground to Air turrets, once inside the gates, attacking team would need to disable a few grund to air turret, in order to be able to bring troops in using the Dropships, this would be optional, some teams might just chose not to use the dropships all together and bypass this objective.

For these battles(the NULL idea included) I'd also suggest NO clone count. Either defending team destroys ship, or attackers take the HQ, this shouldn't come down to kill count, more so to best tactics/defense v offense.