Some of this stuff takes a lot of programming, but a straight up lock, even in the setup phase seems easy.

Boiling oil really should make back in too. The crispy critter animations are already done and in use. How hard could it be to add? The gate would become like the stakes long-bowmen deploy, you just would not charge it.

It might make siege trains worth hauling around if trying to simply knock down the gate and charge was a fraction as expensive in good troops as it was historically.

This also simple enough for the AI to use well.

Then there are the moats....... :)