The Heroes Round Table

I tried to get into this game several times, it seems like the perfect game for me. I really like the concept, but I seem to have an extremely hard time finding basic viable strategies. Call me stupid or whatever but I didn't start playing HoMM until number 5, then bought 3 and 4 to try them as well...

The tooltip for this states that it provides your armies with a ballista, that sounds like it should be a free ballista. Am I incorrect on this, because I can't figure out how to acquire said free ballista...