I think the reason we don't see a lot of war machines and siege engines in the game is because it is implied they are built on site.
A 1000 militia don't deplete a castle's defense by standing outside. I always imagined the idea is if there are more people besieging, there are more people building and utilizing siege equipment, which is why a city falls faster to 1000 people than 100 people. If not for siege engines, really it wouldn't matter if there were 10000 people outside unless the idea is they are chipping away at the stones with their swords or standing on each others shoulders to get in.
What has really always bugged me is the idea of besieging underwater fortresses. Really, can't swim over the top of that kelp fortress? come on.
If by war machines you mean something else, then i apologize
