I have a Big Bad Oppressive Government (BBOG) semi-inspired by the Roman Empire with the conceit that "all portals lead to Mul Khuldir," i.e., that they've figured out super-fast logistics based on a central cluster of permanent Teleportation Circles that can be connected to scroll-cast Teleportation Circles near the battlefield ( (Every warmage has Sending to coordinate and then a few Teleportation Circle scrolls to bring in the first support platoon and additional warmages). The Great Mustering Hall in the ancient Dwarven citadel of Mul Khuldir is where they've been refining the martial exploitation of portals for hundreds of year and the Magisterium of Arcana has blah-blah tried to suppress all non-government-sanctioned learning / exploitation of magic.

Anyhow, the party has now made contact with the potentially-revolutionary enslaved dwarves inside the citadel and I'm fleshing out what the defenses of the citadel might be. Since portal magic is central to the BBOG and the citadel's near a thousand years old, I've wondered if one thing you could do is: 100' vertical shaft with a permanent teleportation circle on the ceiling and floor. The floor's teleportation circle is connected to a vertical (or even pitched up) circle on the walls. You drop a rock or cannon ball down the shaft, it emerges horizontally at speed.

Thoughts?