[Legbreaker]

Layers on top of layers given time and resources I would hope.

I don't feel placing barriers at the base of the wall is going to achieve very much. Having been involved in the construction of a number of earthern dams over the years, I feel I can say with some authority that you can create a very stable earth slope of 45 degrees or so with nothing more than a bulldozer. The vehicle uses it's own weight to pack down the earth as it goes. The earthern wall could be used without the containers also - just means a bit more earth to shift to achieve the same height.

The earth would of course come from a ditch dug around the outside of the defences, effectively doubling the height of wall that would need to be climbed, and creating a very effective anti-vehicle barrier to prevent ramming. One section of earth could be left in place to allow friendly vehicle access (perhaps via a twisting path through various obstacles such a posts driven into the ground), or if resources allow, a heavy metal drawbridge could be installed (an M-60 AVLB would be an ideal short term measure for this).

Once the heavy earthmoving is complete, the wall can be reinforced with barbed wire and mines, with further wire, mines and vehicular obstacles sited further out. As others have indicated, fire positions featuring automatic weapons covering every inch of the nearby terrain are vital. A mortar or other indirect weapon(s) would be very useful sited inside the defences.

If you were feeling particularly frisky and had the resources, a couple of ramps inside the walls could be constructed to support armoured vehicles enabling them to fire from hull down positions over the top of the defences.

All that would make the base almost impregnable to an assaulting force without indirect or aerial fire support (or the time and inclination to dig a mine shaft underneath).