/1/ The 4' X 4' X 6"thick block of styrofoam is inset flush with the ground into a lip around the entry. This foam block is strong enough to carry a layer of dirt to resemble the ground around. It's insulated to provide no IR signature. It's light to lift out or push out easily.
/2/ The ventilation exhausted remotely, probably into a hut to disperse the heat or chem signature.
/3/ The entire hole could be dug by about two men in less than a day. This means only your most trusted confederates need to know where you are, unlike Uday and Qusay in the mansion.
/4/ One taxi cab for road travel, and skiffs on the river for dual methods of supply, movement or rapid escape.
I'm betting he had at least ten holes just like this one, down the the fans and foam blocks.
As far as comms, no doubt he avoided electronics for good reason! The plus side of this is if he had to keep track of messages, we may have scooped a valuable bundle of paper intell with the names, numbers to call etc.
Reports are already coming in about apprehensions of high level baathists still at large due to the papers with Saddam.