Max DPS is damage divided by cooldown at the last upgrade. For a tower with a magazine (Gunner, Pistoler, Sniper) the reload is spread over the magazine, so it's what the tower keeps up rather than its burst. It doesn't count extra targets, blast radius or abilities, which is why a tower like LAZER, whose damage is all in its abilities, shows none. Per 1K cash divides that number by what it costs to place the tower and buy every upgrade.
Every tier of a tiered tower is its own row because each tier is its own tower in the game files, with its own upgrades. The ones players trade link to a page with the full upgrade table, the traits it can roll and its skins. To see a trait's effect, use the damage calculator.
Questions
What's the strongest tower in Retro Tower Defense?
By damage a second at max level, Snowman IV (Heroic) at 300, then Gauntlet III and Snowman III. Towers that hit several enemies or damage with abilities do more than that number shows.
How many towers are there?
107 in the game files, counting each tier of a tiered tower (Headless I to IV is four). That's 21 tiered lines and 60 Heroic towers.
Which tower makes money?
Tycoon: at max level Tycoon makes 1,500 cash a wave.