Portcullis' (portculli?), gates, siege ladders/towers, ballistae, and boiling liquids.

[Chuck]

So this definitely falls on the suggestions part of this discussion. So, here we go.

portcullis...es?
so basically theyre the large gates that are raised and lowered on the walls of castles. In this game, they could take awhile to raise, take a while to lower, but have much more health than the reinforced door. Takes up 4 tiles width, and the gate is 1 wall tall, and the mechanism sets on top of it, which also is the height of 1 wall. so in total, gate+mechanism is 2 walls tall.
gates
basically the pre-requisite to the portcullis. Less health than said portcullis, but cheaper, quicker to use, and its like a door, but bigger(2 wide, and you can put 2 of these together like a double door.)
siege ladders and towers
it only makes sense that with better ways to cower behind walls, there should be better ways to get inside of walls. I know on the road map there are "mountable siege engines" but right now, that may become an update a long time from now and it doesnt say which siege engines, so... yeah. Siege ladders. Enemies can carry these up to walls and use them to climb up. siege towers are more advanced. If you shut everyone inside, the enemies could just chop a few trees and make a siege tower(or they just spawn with it). This can be the type where people can climb up onto the ladder and get onto the walls. ooooorr, ive heard that the more "historically accurate" version of a siege tower which is just giving archers a way to fire over the walls or at inside defenders. But i dont care about historical accuracy, so... ¯\_(ツ)_/¯
ballista
mounted giant crossbow with a lot of range. But you cant move it, because you build it on a tile. Pretty simple. Could also have ballista I and ballista II. 1 gives you the light ballista, which has good range, and a reasonable rate of fire. 2 gives you higher range compared to 1, lower rate of fire, higher damage.
boiling cauldrons
building cauldrons on walls, and then you can put fluids in the cauldron, then pour them on enemies after a good heating. yes, this could be oil, but i dont know why boiling water wouldnt work. or sewage.