A fun idea, however, a sidescrolling tower defense game will have some problems that a top view one wont. For example, having to leave my incompetent troops on the front line while I go all the way back to the shop for backup. Instead, I'd suggest having the ability to call for help anywhere on the map, but the kicker is that your aid will still spawn at the camp, leaving you to fend for yourself until they slowly hustle to your location.