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  Burning their houses is all well and good, but it doesn't exactly mean you have gained access to their city just yet. The fire you had rained down on them could only be repeated a few times more, and then you would be left with an empty shell of a city.
No, slighting their houses was fun and all, but now you shall focus upon breaching their walls.
The walls of Ullsby are old; some sections are modernised, but the walls themselves are possibly from the Viking era, meant to ward off raids by the elves and other such people who aren't capable of building modern siege engines just yet.
Older, traction-based siege engines, like onagers and mangonels, would not have been able to make a dent, but a counterweight-based weapon like the trebuchet could probably do some serious damage to those ancient walls. With the right aim and enough ammunition, Ullsby's defences could be breached in no time.
The trebuchet you had brought in, made from Mithradian pines you had brought with you from the crusade, took a few days to build. It, like the ballista, could be disassembled when you pleased, but the trebuchet was far, far larger than the ballista.
It took about six carts to transport it about, and it could be built up in a month; it wasn't as large as some of the trebuchets you heard other monarchs were building, but the crew and the army in general had nicknamed the weapon Steinwerfer, the rock hurler.
The rocks were thus loaded into the trebuchet's sling, the crew pulling back the counterweight with all their might, preparing to sling their load against the city walls with a terrible speed that would hopefully destroy them.
<span class="mu-s"> Roll a 1d100. </span> [DC 75] A medium difficulty of 60, +5 due to outdated defences, and +10 due to having an experienced crew.