A while back, I saw a post wherein someone made functional machicolations using a complex setup of building pieces. It provided a very narrow opening thru which to aim, and it seemed rather difficult to see out of them as well. While those definitely provide better cover than what I’m about to show you, for those playing PvE or RP, the visibility and aesthetics of these alternate versions may be preferable…
- Snap square ceiling tiles overhanging the edge of your walls where you want the machicolations to go.
- Add crenelations to the outer edge.
- If desired, add awnings above, or inverted sloping wall pieces below.
- Side-grade the ceiling tile into a hatch frame (by clicking on the ceiling tile while you have the hatch frame active).
- Place the hatch door into the frame so that it opens away from you.
- Disable auto-close on the hatch door.
You should now have something that looks like this:
Once completed, you’ll have visibility to see if attackers are at the base of your wall and can flip the hatch door open to shoot at them. Additionally, the crenelations fill in the gaps between raised hatch doors, helping to provide cover.
Note: The reason we use ceiling tiles first rather than the hatch frame is because some elements have an “order of operations” that have to be followed in order to get pieces to socket into place properly. IME, awnings, and inverted wall pieces were the most troublesome.
Additionally, gate frames are particularly finicky, but this trick can be used on them provided you first snap some ceiling tiles inside the top edge before extending outward, like so:
And yes, the structure will survive upgrading and server reboots. 
.)