We played a championship on Dungeon, and one of the 5 maps was Beltway... I actually took 1st place there ;)
So, everyone was surprised how it was even possible to place Green Dragons there in the first week :)
They even asked for a step-by-step guide - if anyone is interested, search for it :)
So, nothing is impossible.
And about the first day - if the starting Castle is Warlock and Gargoyles are built... how many more units and Gargoyles will we get? ;)
I'm not saying it's impossible. I'm just saying that not every map allows you to do it :) Specifically, to break through with a lot of units on the first day.