A) DON'T MESS WITH THE MAMMOTH CITY EARLY IN THE GAME
1) He is a though-though defender. Even if you can kill it, it will cripple your initial stack.
2) He resides at the back city. Even if you get the city, you can't expand from there. It will take your hero 2 turns to reach there and two more turns to come back to where you started
If you want to get mammoth early, plunder the cities and buy yourself one. Or just wait until your first hero expands and try to get the Mammoth city with your second-third heroes. Sometimes there is another power unit there instead of a Mammoth but the same deal goes.
I would like to continue this key to success series. Maybe other players may contribute to this or start a new one giving hints about other maps.