Ah. I did not mean that the chained animal needs to be on the right of the chain always.
Only in this particular setup.
For, when it is at the chain, or to the left of it, it is more than 4 spaces away from the spider, and the spider won't start shooting.
Luckily, it seems chained animals are not scared by the GCS, and will wander happily in it's sight range.
Also, I don't tame the spider. It is harder to chain up a hostile creature as bait. I just put the spider in a cage, build it in place, hook to lever and release. Sometimes the mechanism gets in a door opening, resulting in the immediate recapture of the GCS.
To prevent that, make the enclosure the GCS is in 1x3, and build the cage in the middle.
Do forbid the mechanism and the cage after release however, or FUN will happen.
In this setup, a 2x8 raised bridge would have been better than the 2x4 I built.
My weaver is scared away from the tiles that are diagonally next to the fortifications. A 2x8 bridge would close of sight completely.
EDIT: This is a setup I have prepared to make a webbed corridor for sieges to come in through:
Further to the south is a catapult. The spider will be inserted behind the fortifications to the left.
It will shoot webs at one of my soldiers, who I will maneuver from north to south, over the 3 bridges on the right. I can make the spider go up and down it's hallway by toying with the pet-passable settings on the doors.
When the hallway is webbed, I retract the bridges the bait-soldier walked over, leaving only webbed path.
I then recapture the GCS, and put build it's cage back in the hallway. When there are no archers with a siege, I can release it to spray more webs as the siegers struggle through the corridor.
Traps can also be webbed over this way, to be able to trigger on [trAPIMMUNE] creatures.
Downside is, they will trigger on your dwarves too...