Enemy (building)

I have a building and i want it to instantiate enemies every 15sec and if i shoot that building it dies and it stop instantiates.

Thank you! P.S. : this is the last part of my game if i can get this to work :)

Really quickly...

Spawn enemies with a timer:

    function update(){
     if(!blownUp){
       timer -= Time.DeltaTime;

       if(timer < 0){
         timer = spawnTime;
         // instantiate a zombie
         }
       }
    }

// hit detection or how ever you've set damage to work
function OnColliderEnter(collision : Collision){
   if(collision.gameObject.tag == "bullet"){
       hp - 10;
       if (hp<=0) blownUp = true; // blow up
   }
}