Now, I will admit, Maldita Castilla is a game I only discovered tonight from a stray youtube comment on a "Ghouls N Ghosts" playthrough, but, only on the second level, I can already say that this is without a doubt one of the best retro-tribute games I've ever played and one of the most uniquely themed.
That being said, I have NO idea how this game isn't even a tenth as popular as the likes of Cave Story and La-Mulana.
To explain the game in brief, MC was designed with retro in mind from day one; the display window the game runs in emulates a CRT arcade screen, and all of the music was composed using the same emulated Yamaha FM chip used for lots of late 80s arcade games. The game borrows heaviest from Ghouls N Ghosts, but also has elements of other late 80s arcade action titles like shinobi and black tiger. However, what makes it interesting is that the entire setting is based around traditional spanish legends and monsters. Spritework is GORGEOUS on everything and, while limited to expected colors/pixel sizes of the time, manages to look super smooth and great in every aspect. There's plenty of level variety to keep things interesting and a fair share of bosses.
Oh yeah, one more thing...
IT'S COMPLETELY FREE.
So yeah, here's the gameplay trailer.
Here's the site to download it.
http://www.locomalito.com/maldita_castilla.php
If this is your kind of game too, you know what to do.