The towers without any upgrades. From left to right, archer tower, barracks, mages and dwarven bombard.

From Left To Right: Archer Tower, Soldier Barracks, Mage Tower, Dwarven Bombard.

When You Start The Game, You Begin With 4 Tower Types. As You Progress In The Game, You Unlock Upgrades and More Advanced Versions Of The Starting Towers (See Picture).