I know im not the first one to come up with a killcounter for your defenses but my idea is to give defenses ranks or new names for every 1000 kills (could be more or less). Just like the strange weapons in team fortress 2. First your mortar is called: "Mortar" and after 1000 kills it becomes: "dangerous mortar" and after another 1000 kills it becomes: "nasty mortar". It rewards players who place their defensive buildings strategically with those cool new ranks/names.
