Apparently Nue’s final card in TD is one that gets harder over time. It gets pretty interesting towards the end and it’s a really fun card.
The Ultra patch also does wonders for this card and makes it quite challenging.
Ultra Mode is a patch that you can apply to the game. What it basically does is double the amount of bullets, double their speed, both, or something else. This makes for a fun challenge most of the time and sort of gives you a difficulty above lunatic.
There are patches for almost all the Windows games and they can be downloaded here: