[I assumed The past was being ignored, and all was as future~]
Eric shambled groggily up to the surface, where the carcass of some great abomination lay slumped in a pond of blood, being dismembered by a team of butchers. He'd slept right on through the attack this time, and was woken by one of the dwarves when they came to tell him that the gate had been blown through again, by another damn mutant. Altogether, three of the things had flown over the walls, two had jumped, three had climbed, one had tunneled underneath, and collapsed a section atop itself, where it was smothered, and five had simply gone through the front door. While it was closed. It was the larger ones that seemed to prefer breaking things whilst making an entrance.
He had, in Wierd's stark-raving-mad absence, been forced to do any engineering on the defenses entirely on his own. Engineering wasn't really his thing, so his plan for dealing with airborne/leaping threats was simply to station mid-sized ballista turrets on the ledge around the seventh story of the rather tall central keep. The marksdwarf squads had a slightly busier schedule, which included more "shoot at anything freaky looking coming out of that wreck" and less "go to target practice." Or maybe it was more "go to target practice." But with larger-than-average mounted crossbows.
He hadn't been able to figure out how to prevent the abominations form climbing up over the walls, besides smoothing them with glass, and waxing said glass, on a regular basis. Those beasts had been insects, spiders, and a local gecko. Somehow, being about the size and mass of a horse didn't prevent them from scaling the three-story walls.
His plan for preventing them from bashing right through the gate was thus: Turn the gate into a trap. Now that the gate was down again, and the stonework around it also in need of replacement, he would be able to implement the plan. The gate itself would be replaced as per normal, but there was a catch: when a beast struck the gate forcefully enough to crack a special lock, it would trigger a set of heavy steel pikes to be propelled into it from a spring-loaded mechanism above. There would be a set on either side of the gate, interior and exterior, such that the beast would likely take multiple injuries. Beyond the gate, inside the compound, he would place a cannon. A 4-inch diameter rifled steel barrel, reinforced with expensive adamantine struts, breach-loaded with an artillery shell, mounted on a rocking chasis that could absorb some recoil, and hopefully avoid busting the gun. It was all powered simply by black powder he'd mixed up himself, though, and it had been rather dodgy in that sense. If the guards so chose, they could fire the cannon at any point the beast was in front of the gate. The projectile would more than likely go straight through the wooden door and still have momentum enough to kill or injure the beast. Or it wouldn't go off at all, perhaps.
Within the hour, he had a few dwarves to help prepare the defenses. At some point, he intended to make a damn golem or something.