A gamemode of 16 for KOTH entities (also needs to be in the worldspawn). To make things simple, I added a "hill" entity. You can also use a flag, either as an indicator or for the player to touch. I generally place a flag in the center and use the hill entity to expand the touchable area. Note that you can add trigger entities to .ent files using "mins" and "maxs", but you will most likely need to set a "model" to keep the game from crashing (I've fixed this for a few entities).
So you could do something like,
{
"classname" "flag"
"origin" "0" "0" "0"
"gamemode" "16"
}
{
"classname" "hill"
"mins" "-128 -128 0"
"maxs" "128 128 64"
"model" "models/items/ammo/tris.md2" // to keep it from crashing
"gamemode" "16"
}
You can also do some creative things like add "grabable" "1" to a flag, and be able to do weird hybrids of KOTH/CTF.