Sounds like a cookie-cutter desing, tbh. But if you’re struggling with inventing upgrades, consider some or all:
• look into medieval engineering and siege weapons to get a sense of what was actually used vs what was created and failed - especially the failed constructs could be useful
• have a look at the custom stuff people build in Besiege. You may get handfuls of whacky DaVinci-like inspirations while laughing your socks off
• either incorporate or straight up neanderthalise modern military into the design. How would a 14th century C-RAM look and behave like. Homing rockets? Think materials they had commonly at their disposal.
• add magiks into the mix
• there’s also the question of the overall theme one may need to stick to for consistency. Nothing stops you from making a giraffe-tower that spits out acacia spikes. An elephant artillery that hurls boulders. Depending on how your creative juices are pumping, it might be easier to upgrade animals rather than lifeless and inanimate stone & iron buildings
• ask AI generator to keep dreaming up siege weapons, once something catches your attention, make it in Blender
• once you have sketches / renders / models (or even short descriptions of how stuff works in-game), start a brainstorm thread either here or on reddit, ask folks to give those upgrades fancy name, something that ties into their functionality.