These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Heavy Killers. Like their name suggests, they're very fast and are equipped with machine guns that screw up large characters. Also they'll always be in at least pairs.
Game Breaker: With the "improved sell-back" and "cheaper Ghost Towers" upgrades in Tower Defence 2, you can actually profit by building and selling tier one ghost towers. Admittedly it takes ten iterations to afford a single basic tower, but since the game won't start until you let it, with a lot of patience and mashing the 2 and S hotkeys, you can garner a substantial early advantage.