My simple guide (mostly images) for all fellow Engineers and their precious Ballistas, some interesting spots to place your Ballista and give yourself and your team certain advantages in the match, if play right, you could singlehandedly hold back enemy's advance and support your Frontliners extremely well.
We start with Red team which is arguably the unfavored in most of the map for some mysterious reason.
1st are these spots on the roof of the burning house near Red's 1st point. These are decent spots to place your Ballista and pretty useful in Invasion mode, because the enemy rarely reach this house and focus more on their objectives. You are perfectly safe from Horses (same as all high places) but you are exposed to archers and firebombs if they managed to get close, luckily the armor box is right below your feets.
You can actually put your things on this "structure" (no idea what it is XD) but to my experience these spots aren't particularly useful and risky too, try yourself in certain situations.
One of my favorite spots on this map, you can provide support for your mates and hold off enemy's advance very easily, easy to go to and place or fix your Ballista, most of the time your are safe from archers because they usually stand on the fences of the Camp thus you are out of their vision, 0/0/0 roamers and bomb spammers can only approach you from the left which is entirely in your FOV, the armor box is pretty close too, be aware of horses though.
A decent spot when Red has been push back to their camp, you can also provide suppressing fire to the middle point albeit difficult to be accurate due to the distance. In order to build the Ballista you have to drop yourself down then go back up.
Now onto Blue, these are pretty standard spots to defend your Camp after the stable is lost, personally I switch to normal melee fighters because when it reached the point where your team had to retreat here the game is almost unsalvageable.
Another excellent spot for you to support your team, the FOV is incredible, pretty much safe from roamers (unless your mates are blinds or trolls) and firebombs due to the height, even if archers got a hit or two on you, you can always jump down the fences to regen, the only serious threat though is the catapult XD but as I recall, the guy on Red had to shoot about 10 times to hit this roof which indirectly save your teamates ton of lives XD. To reach this place climb on the fence on the right then climb on the roof.
Standard spots to defend the stable, the enemies will have to line up to be your target dummies if they want to push the cart XD which is always a good situation to be in, not entirely safe but if utilize at the right time can be significant to the outcome of the match, the serious downside is you are free food to the catapult if it's still alive.
A extremely good spots to defend the center point albeit not safe at all XD, exposed to everything, it's best that you jump on the Ballista right before the enemy reach the point to not be overwhelmed and pick them off 1 by 1, teammates are absolutely needed to defend the points with you, the more of them the better distraction for you to shoot. The same can be said when your team is attacking, stay behind teammates and quickly build it to support them and drive back the enemies.
A near useless spot but let's just include it in for the sake of professional XD
Lastly, these spots obviously has extremely good FOV but their utilization is surprisingly low, archers can easily disrupt you and you have no where to hide but jump back down which is very inconvenient, on the other hand if you have competent archer mates on your team, help them climb on these towers to take advantage of the extreme FOV. To climb on the top of the tower, you need to build 3 spikes as in the image, you can do the same to the Eastern tower, after those tower got bombed, you can build 1 spike to climb on them and build your Ballista, this time is safer and actually viable.
Map: Mountain Peak
These are not all the spots, I just posted some of them for you viewers to form some ideas about the "philosophy" behind a good Ballista placements.