So, here're the construction snaps:
Randomly shaped cakey bits on the base board, glued down with buttercream.
Covered with fondant.
Then green royal icing scrub, brown piping gel mud and cookie crumb earth.
Airbrushed to give it all a bit of tonal definition.
Meanwhile, carve the cake into a tank shape (I chose a Sherman and had a look on-line to see what they looked like), crumb coat and cover with fondant.
You need a fairly firm cake for this, if it's too soft you wont be able to carve it well enough and it won't support itself.
Carve and cover turret separately. Glue turret onto main tank body with royal icing.
Cover a sturdy straw with fondant stuck down with edible glue. Allow to harden.
Start to add fondant detail.
You need something sturdy to take the weight of the cannon barrel. I used a wooden skewer, then some narrow straws split to fit inside each other and pushed these into the cake quite a long way to allow for leverage.
I slipped the barrel over the narrow straws and used edible to glue to anchor it.
For the tracks, I made a chevron design.
I glued the tracks on and dusted the tracks, wheels and gears with edible metallic dusting powders then airbrushed the whole tank.
The final details...
...then place the tank on the base and have a nice cup of coffee.