Custom Missions are additional missions for KKND2: Krossfire.
custom campaigns are basically just custom missions that you can play as a normal campaign. enchanted, isn't it?
Click here for the multiplayer map collection
a small selection of demos from KKND / KKND2: Krossfire and no you can't build ballista towers
The KME is a tool to create custom missions, custom campaigns and multiplayer maps for KKND2: Krossfire
Krossfire Mission Editor