RTS CD

I`m designing the collision detection system, it`s a basic thing but I forgot how it`s done (I had it once working). Do I need to check three out of  four tile corners to get a collision?

CBody CollisionBodies[100];
int BodyCount =-1;
VOID AddCollisionBody(int posx, int posz, int tilewidth, CBody * Bodies)
{
BodyCount++;
Bodies[BodyCount].nwx = posx - tilewidth / 2;
Bodies[BodyCount].nex = posx + tilewidth / 2;
Bodies[BodyCount].sex = posx + tilewidth / 2;
Bodies[BodyCount].swx = posx - tilewidth / 2;
Bodies[BodyCount].nwz = posz + tilewidth / 2;
Bodies[BodyCount].nez = posz + tilewidth / 2;
Bodies[BodyCount].sez = posz - tilewidth / 2;
Bodies[BodyCount …