From the steam page:

So, the Ratz-Server software is included in this update! Using it, you can create your own private server on one PC that holds up to 100 players… perfect for LAN parties! You find the files and instructions on how to use it in the installation folder of Ratz Instagib.

Also in this update, I added the other two prototype maps. I think they still hold up quality-wise… enjoy them! Map 1 and Map 2.

• Spectator Mode

Now, you can spectate on matches as well as play them! Hit ESC at the start of a match before spawning and press the Spectate button. Please note, only two people can spectate on a match at a time right now, and you can only switch to Spectator Mode before a match begins.

But wait… there’s more interesting stuff in this update too!

• Custom Crosshairs Look for the "CustomCrosshairs" folder in your Ratz installation folder. The game will read all ".png" files in there and put them into the Options menu to choose!

• Crosshair Colours Change the colour of your crosshair to anything you like!

• Spawn sounds Other players now spawn with a sound effect, so you don't get surprised if someone spawns right behind you.

• Tweak: Reduced the amount of wins you need to unlock the Freeze-Fragtype to 50.

• Access other players’ Steam Profile after the match