Version 19 introduces 3 more fully completed levels:
Episode 3 Secret: Alien Dimension with MeowMeow wreaking havoc at the alien homeworld.
Episode 4 Level 1: Castlemania with spooky atmosphere and silly references.
Episode 4 Secret: G8 To It with a stupid amount of monsters, big weapons and varying biomes.

If you can't find the secret level entrances, they are selectable from the Story Mode menu for quick access.

There's also a major improvement to the game performance overall, so there should be way less stutter and no little freezes every now and then. Using autosaves might still cause tiny slowdowns. Gamepad support is better in menus and during gameplay. Menus also got some cleanup and polish. Intro screen got nice fade ins to the visuals and audio. Looks classier overall.

Earlier the game defaulted to Borderless Windowed, but this was causing some stutter, so if you want smoother performance, be sure to use Fullscreen video mode.

Here's the full changelog:

LEVELS:

• E3S1:
  The level is now fully complete
  There's a big new area in the beginning of the level!
  
• E4L1
  The level is now fully complete
  A whole lot more areas!
  
• E4S1
  The level is now fully complete
  There's one more big area to explore!
  
• E1L3:
  The rotating thingies have their vertices snapped to grid
  There's a button in the sewer next to the assault rifle
  There's some blue decorative lights near the stairs and teleport at the top side

PLAYER & WEAPONS:

• Fixed some dumb issue with diagonal movement with the gamepad
  So playing on the gamepad doesn't feel like being a little drunk
  
• Hitscan weapons that displayed explosion models no longer spawn an explosion mid-air close to player if gibbing an enemy
  
• Added particle effects to Plasma Grenades and Mega Cannon Plasma
  
• Grenade Launcher & Black Bombs:
  The clusters explode in mid-air after a very short duration
  
• Railgun:
  Uses only 5 ammo per shot
  No damage reduction when it pierces enemies
  
• Plasma Grenade:
  They now have a little shock blast effect
  Only 4 smart plasma balls

POWERUPS, MONSTERS & MISC:

• Catnomicon Combat gives a 20% damage boost, earlier was 25%
  
• Added a wall candle object
  
• Some adjustments to how kill enough monsters counter triggers work on Nightfeline and Mäyhemicus Catus

MENU, MEOWPEDIA & HELP:

• Cleanup on the menus!
  
• Using the menu with a gamepad have more standard controls
  
• There are helper graphics for using the menu at the bottom
  
• Version number is visible in all menus
  
• The intro screen has some fade ins to make it more classy
  
• Removed the menu ambient sounds
  There's already enough ambient sounds coming in from the levels
  
• Meowpedia & Help have titles
  
• Player, Weapon and Monster stats are recorded to cvars to make it faster
  Reading/writing to text files during gameplay caused occassional freezes and other minor fps drops

SETTINGS:

• Due to popular demand, the voice volume setting is no longer available!
  
• Fixed Yaw Scale and Side Scale naming in Input Devices menu
  They were named incorrectly, should have been the other way
  
• Gamma slider numbers are no longer flipped
  
• Ammo capacity setting now has an Unlimited option
  
• Autosave interval adjustments due to also causing some fps drops
  Minimum interval is 30 seconds
  New default interval is 60 seconds
  0 disables autosaves
  
• Language menu and other languages than english are removed
  The translation system was also causing occassional freezes and other minor fps drops
  
• The story mode menu allows selecting any level without having to reach it
  So if you really can't find the secret levels, you can still play them
  Also the multiplayer menu allows selecting any level anyway...