I would actually reduce the attacking range though. While trebuchets and the likes are what drives a turtling player out of their base, it does come with its set of drawbacks: it essentially becomes a building. If you have a mobile unit that can fire halfway across the map, even moving slow is not a sufficient drawback. You need the opponent to know the origin of the fire, and that they must strike this position. If the enemy can move back too easily, then there isn't any right play, and the opponent will just build the SAME unit.
You want units to counter one-another, not be dominant.
Actually it worked out really well. it would take a while for the shots to get to you, and resetting for a second shot takes a while. Plus you have to have visibility there (like a Terran ghost setting up a nuke) if it was your base, then you need to look for anything with visibility near by. If it is against your troops, you just move them, even a little, and the Artillery would miss. (not an area attack, only a targeted impact) second, you didn't get to see where they where from, which created their own issues, but you could fire at targets you couldn't even see either. For instance, you could target a black area of the map, and the shots would go there. You would have no idea of a successful hit or not, but it worked.
Because the Artillery were about half the speed as anything else, and took so long to fire, it was great in bulk when well defended, but easy to evade, if the target wasn't hit by 10 of these at once. Regarding the origin, you know the direction they came from, and would start targeting with flying units that could get there pretty quick. The Artillery could not shoot flying units. In the game play, I enjoyed this mechanic and found it well balanced in the game.
Look at the gameplay mechanics, and insert some kind of infantry/pikeman/archer/cavalry "rock paper scissor" dummy units and see how they interact.
That's a good idea, and in general I would agree with it, but I've toying with a different idea now. So far, Star Craft and others are based on rock scissors paper technology, but the handling of each is almost identical. I've been considering different modes of game play depending on the race. So its not just weapons/defense balancers, but also tactical balancers, in the sense of how things move and work together via AI, and Player control.
Over all, you sound like you have very sage advice, and have some good experience in the RTS Area. Thanks for your insight.