In Weaponizing the return-energy channel of a one-way portal, I describe the function of a hermes portal, which are magical devices that, when activated by some command, create a connection to a linked portal device. Portals allow objects to pass from the source (the portal device that was activated) to the destination device. However, objects cannot travel back from the destination device to the source device without the destination device being activated as a source. The activation command is made specifically for the portal when it is created and is often effectively impossible to derive from a portal (active or inert) without destroying it. It is possible for an object or person part-way through a portal to retreat, as long as it has not passed completely to the other side.
When a portal is activated, energy can travel in both directions through the portal, both electromagnetic radiation and sound energy, though the gases that carry the sound energy do not actually pass through the portal. This allows the person activating the portal to more easily determine if it is safe to proceed through the portal prior to actually doing so and allows a person on the source-side of a portal to communicate bidirectionally with a person on the destination side. This is the return-energy channel.
Physically, a portal device is often a physical doorway or tunnel, or perhaps a ring, maybe like a stargate, but it could be much more lightly constructed, possibly as little as a stiff, shaped wire. A portal functions from both sides simultaneously without collision of transmitted objects within the portal.
Now, following the suggestion given by BMF in the accepted answer, my characters have set up twenty capitol Fulminium laser-blasters pointed into the portal, ten each side, mounted so that they will jitter in random patterns across the mouth of the circular portal.
A Fulminium laser-blaster of this type emits ten (10) 8.3 MJ one-millisecond (8.3 GW) pulses of orange and UV laser light per second, each pulse carrying an energy equivalent to nearly two kilograms of TNT. In addition, these weapons emit copious amounts of electrons, which are typically discharged down the ionised channel of air that the laser beam creates in the air between the weapon and the target. The Fulminium rods in the lasers are the weapons' atomic power sources, and contain enough energy to fire continuously for years.
Since the hermes portal which is the target of these lasers is receiving an incoming connection, the light can pass through the portal but the electrons from the blasters cannot, but instead remain on the surface of the portal field, accumulating a charge of up to a billion volts. However, the effects of the charge can pass through the portal's return energy channel, repelling negatively charged objects, and attracting positively charged objects. Any positively charged objects, such as ionised dust or gas, can pass from the source side to the destination side, capturing some of the electrons on the destination side of the portal field, resulting in free electrons on the source side.
On the source side of the portal is a large walled city, much like Carcassonne in that it is entirely land-bound, and sits on a large hill-top, with the palace about 500 meters from the outer wall, with suburbs outside the outer wall:
However, the city is much larger, on the order of the size of Byzantine Constantinople, roughly six kilometers across:
The following diagram shows the position of the portal and the ranges to the relevant structures. The portal is placed 500m from the city wall, on level ground in a square in the city's outer suburbs, one side of the portal facing directly toward the palace. The diagram shows a scale representation of the hill upon which it is built. The hill is composed of earth and granite.
The portal is about 3m in diameter. The lasers are placed from 0.1m to 1.5m from the bottom of the portal. The lowest lasers are set to sweep about 1° below the horizontal for a small portion of their movement, but all the lasers are set to sweep up to a maximum of 6.5° above the horizontal. The lasers will sweep an arc of up to 179.75° to either side of the centre of the portal (so that they will miss the edges of the portal device, which is quite thin) on both sides of the portal, ten lasers on each side, each pivoting randomly and independently within these limits.
The instant the portal is opened, the lasers begin firing, and continue to fire continuously for the ten minutes that the portal remains open, sweeping the area around the source portal as described above. The lasers are suitably heat-sinked, and won't burn out or reduce their rate of fire due to their own waste heat.
In the vicinity of the portal, waiting to pass through, are 5000 infantry soldiers wearing late medieval plate armour, and six solid steel automata ranging in volume from 100 to 500 liters, which may simplistically be considered to be ovoids twice as high as they are wide.
The buildings in the vicinity of the portal are late medieval timber, stone, lath and plaster structures with slate roofs. The buildings inside the walls of the city are dressed stone or brick, also with slate roofs, while the palace is heavy fortified stone with copper roofs, like the keep of a late medieval castle.
The city has a population of several hundred thousand people.
My question is: What will be left of the army, this city and its population after the laser-blasters have fired for ten minutes?
Don't forget that the laser-blasters charge the portal up to a billion volts as well as shooting 8.3 MJ / 2kg TNT equivalent laser pulses 10 times per second for each of 20 lasers, for a total of 200 pulses per second emitted in a fan from both sides of the portal.