At the start of the video, when you were moving the turtle? You can see a monster units sight range on the same screen as its movement. If a monster unit can't see you at the beginning of its turn, that unit won't move to attack you.
It might move, but by some quirk the Monster faction AI won't attack a unit it can't see at the start of its turn. So even if it moves into sight range during the turn, you are fine.
Re perk buildings and the Capital, I've found that if I concentrate the capital for the "non-magic" line, you can quickly get the Barracks (Drilled purchasable perk), Smithy (Fine Armor), and Training Grounds (upgrade to tier 2 of the non-magic units: Fighters and Archers). If I divert into the magic or resource buildings, I delay upgrade options for my empire significantly.
Similarly, a dedicated "magic units" city will more quickly unlock your magic units. Mana Tap, Alchemical Lab, Shaman/Chateau, Military Academy. It can also get the two upgrade structures that mainly benefit casters and healers- the University and Magic Bazaar.