There's a free for all gametype and that's fine. However not everyone likes free for all. There needs to be at least one team game. And given the content in Spiral Knights I think "Capture the Key" would be fun. I do admit though that there isn't really the population to support two kinds of arena gametypes, but hopefully in the future there will be more people.

I don't know if the devs can do this but one thing they'd want to do is force colors. You know how we can pick a custom color when we make our character (I'm orange)? Well they should force one team to be blue and one team to be red (those are the traditional team colors) for the duration of the match. It would probably be 4v4 max but still allow for 2v2 3v3 (like FFA allows for 4 people).

Each side would have a big key that they own. They would probably want to change the color from gold to whatever their team color is. That, or, you could just have one gold key in the middle and both teams fight over it. The one gold key may be better actually now that I think of it so that both teams can pick it up. With 2 keys there's the issue of it your team can pick up your key or not. To make that not an issue anymore just make one gold key in the middle. It would also be easier because then they wouldn't have to make red and blue keys.

The key would be to open a gate. You'd either make it so you have to open your gate or their gate (one or the other). I think it would be better if you open your enemy's gate. That way you can put like some minerals behind each gate. Maybe 3? And once the gate is down (first off when a gate goes down the key should respawn in the middle so maybe you would put one of those respawn tiles under the key that are like under the fire pots) the object would be to destroy the minerals. Once you destroy all the minerals you win the game.

You would want to make it cost crowns to play and have a net loss of crowns. Then the winning team would get some crowns. The reason is to discourage people from griefing (also for a crown sink). For instance if your friend was on the other team he/she could just hand your team the key. Another interesting reward could be that you gain minerals if you are holding a broken mineral piece at the end of the match (the minerals would automatically attach to your back so when you kill the 3rd and final mineral you can still get the mineral that drops from it before the game is over and if you already have one on your back when you break one the broken one would fall to the ground). This mineral piece would drop from you if you die. This would make a secondary objective that is entirely optional. This would make it so even if your team loses you can still win something.

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tl;dr version: Capture the Key with a gold key in the middle. Both teams try to get it and take it to the enemy's gate. Behind the gate is a mineral patch (maybe 3-5 minerals). You win by destroying all their mineral patches. Players keep what minerals they are holding to put into gate construction.