Settlements are a core of every match, and in multiplayer the games are almost always decided by how the players are able to defend and conquer them. I see a blob of army walking around, destroying/defending together succeed more than any other tactic. In a sense the siege can feel secondary, and even unnecessary, sometimes, as the outcome of a siege seems to be decided before players ever come close to each other's castles, which is unfortunate. So, because of that, I think a most effective way to make players use siege, and also to fight in more different ways than in one big cluster, is to add another way to defend one's settlements - walls!
While the walls surround castles and camps, I think it doesn't fit to simply do the same for the settlements, so instead I suggest to add an upgrade to the faction's research building, that would have two tiers, each enabling to add one wall hub per settlement. The wall hub would connect with simple (not very durable and not mannable) walls to the settlement (and not to the other wall hubs!). The wall hubs wouldn't go far, in fact at half of the screen maximum. Another restriction is that you can't build wall hubs near one another (like at 15% screen distance around one, I mean), meaning that two settlements cannot create a solid closed wall and must leave a passage in between. The settlement walls would be repaired outside of combat for free, but if fully destroyed, the player must pay to rebuild them all at the settlement. If a settlement is destroyed, the walls fall too. If a player demolishes a wall hub, the wall goes down too, and a new one can be build only after a time. Ofc, not all factions would have solid walls: Rohan and Angmar would have thick palisades in place of stone, while the orc factions would have palisades so bad that they have passable holes every few meters - merely deforming enemy formations, but at least with an additional effect of trample slow for cavalry so that it can't charge at full speed. Lothlorien would be functionally identical in this regard to the orcs, except that the walls would be formed out of living green trees, that let them go stealth, if the sections are at full hp (wouldn't that be cool af btw? :D).
What this feature adds to the table is a defense against both the large armies moving completely freely to your castles, crushing settlement at a time, troubleless, and versus melee units surrounding these buildings - they would still be able to attack the farm/slaughterhouse/etc, no matter which side they attack from, but with fewer numbers. Other than the meta reasons and thoughts of increasing siege units use in mid to lategame, I just think it would add a bit of interesting complexity to the players movement and choices in both defense and siege. Many factions would start to treat movement very differently, with Lothlorien elves having an easy time walking around the enemy walls to rush a settlement, and to retreat when needed, which is an opposite to the brutal onslaught of the orcs, who can't stand for such obstacles, either taking time to gnaw and scratch them by hand or getting some siege craft. And when building, these walls are easy-to-learn-hard-to-master-type-of-deal, as in order to actually block a decent chunk of area, player needs to use the surroundings, like mountain ridges and gullies.