Research the elements, draft your towers, and hold the portal against an endless march.
Towers belong to seven elements. Spend knowledge tomes to research an element: higher levels unlock stronger tiers and rarer towers.
Each attack type is strong against one armor and weak against others. Divine armor shrugs off everything except Essence. Check the incoming wave and adapt.
Mass, normal, air and boss waves arrive with random races and special traits. Every 8th wave is a Challenge: it cannot hurt the portal but pays out for every hit.
Towers level up from kills, carry items dropped by creeps, and drink oils for permanent power. Combine spare items with Transmute.
Untrained towers are weak: level 0 deals 35% damage, level 5 100%, level 25 about 360%. Levels go up to 30, and items multiply on top. Every tower starts with 1 item slot and gains one every 10 waves (6 at wave 50), and picks a perk every 5 levels. A tower may carry any number of unique items. From two thirds into the run (wave 80 of 120), spend spare gold on Train to level your carries.