Maggie, having presumably parked Lil Asskicker in her mail bin, comes running out into the yard with more guns. She joins the battle against the guard tower gunman -- who, honestly, is proving to be a worse shot than I am -- to give cover to Carol so she can get up and join them behind the corner of a solid structure. Shooting continues, until they all hear the roar of an engine and pause to listen. Something is indeed rolling up the dirt road at high speed, but it's not Glenn in the pickup -- it's a big, boxy delivery van that crashes through both front gates, smashing them aside. Now you got a breach to worry about, Glenn.
The truck comes to a stop in the yard. Everyone watches, the Governor smugly and the prison group fearfully, like they don't know exactly what's going to come out of there in a minute. Finally the gate-ramp at the back drops open and every walker that the Governor and his men could cram in there comes pouring out into the yard. A slim, obviously female figure in a paintball mask climbs out of the cab and starts shooting. That had better not be Andrea. Rick raises his rifle and pulls the trigger, but he's empty. Worse, Martinez barely misses him again. The delivery driver runs past Michonne and her bus, supplying her own cover fire as she goes.
Carl, Carol and Maggie are still exchanging gunfire with the crashingly incompetent man in the guard tower. Rick yells unhelpfully at Hershel to get out of there, and the Governor just watches the scene in contentment, barely pausing to dispatch the walker coming up out of the woods behind him. Maggie finally gets pissed and takes down the man in the guard tower, who must have been almost out of ammunition anyway. Rick, switching to his service revolver, guns down three approaching walkers before finding that gun empty as well, and has to retreat from the crowd coming out of the woods behind him, obviously drawn by the noise. Martinez falls back, and joins the Governor and their passenger back in the truck, the Governor pausing to empty a clip into the prison grounds for good measure. Hershel has begun picking off the walkers inside the yard because why not, while Rick finds his return route to the gate cut off. Yep, dead Lori turns out to be just as helpful as the live one.
Glenn returns in the pickup, his truck nearly colliding with the Governors' on its way out. With only the walkers in the yard left to deal with, our heroes start shooting into the grass, while Michonne wades in through them toward Hershel, splitting skulls with her katana-fu as she goes. Outside the fence, and Rick is reduces to using his empty revolver as a club and a knife, neither of which do much good against the big dead guy who son has him pinned against the chain-link. Hershel gets to his feet, and Glenn drives the pickup back onto the grounds, looking angrier than ever, but at himself this time. Either he's upset that he was gone at the worst possible time, or, like me, he's disgusted that he thought the Governor would go through all the danger and work of coming at them through zombie-infested tunnels when he could just... do this.