There are actually many ways to siege a town, I'll cover more on campaign battle for the purpose of this question.
In most if not all cases these few points will make the siege much easier
- Use a ninja to sabotage the castle gates before the battle, with the gates open, some units can enter without suffering casualties from scaling the wall. This is especially important for Ashigarus as they have low morale, those casualties together with the charge they receive upon entering will almost route them.
- Attach a Monk/Missionary to the army, the morale boost is much required as defending units will fight to their deaths hen defending the inner most level of the castle. Without morale your units will often route even though you are winning in combat, losing a winning fight.
- Siege from Spring and attack on Autumn, that will result in the defenders suffering attrition but not yours. Watch out for potential reinforcements that'll break your siege.
- If the defender have another army that is not garrisoned, you can optionally attack that unit either making it retreat and no long reinforcing the siege battle or luring the army garrisoning the castle out into an open battle, reducing their remaining forces defending the castle. It is important to inflict maximum casualties on the garrison army in that case so whatever left from the fight will still retreat back into the castle and defend it - regardless they'll suffer attrition from losing the open battle so it'll still be an advantage. Note that garrison provided by buildings will not attack out in this situation so you'd still need to fight them during the siege.
- Enter from gate to minimise loses from scaling wall, if the gate is too heavily guarded, scale walls beside the gate. Usually walls beside gates are low and low wall will cause less casualties scaling, also the units the scale will effectively flank the defenders guarding the gate. Having each unit entering the gate charge towards a different direction will prevent the defenders from flank you.
- Destroying wall/gate will kill a large amount of Archers holding that section of wall/gate. That can be done with siege weapons. Gates can also be destroyed by burning - while it is possible, I do not suggest burning with melee units. Fire arrows that can be fired by archers can effectively burn down a gate.
- Ninjas can scaling walls without casualties and are very effective at capturing, use them to open gates(if not already opened) and capture unguarded towers. Avoid defending cavalry, if need be engage an infantry and force the cavalry to charge you through their own units, reducing the charge damage.
- Monk archers and Chokosabe Daiyu Samurai(forgive me if I spelled wrongly) have 25 range longer than other archers. These archers can shoot defending archer without getting fired upon. Exploiting that in infinite time battles can greatly reduce defending forces left before melee units attack. This also applies to Bow Samurai Hero and the Mounted Bow Samurai Hero which have yet another 25 more range(a total of 200).
- Despite the description saying it being weak against archers, the Naginata(?) Samurai can have a total of 14 armour if upgraded by Amoursmith(+3) and Armoury(+2). These units will suffer much less casualties than others under fire, though there is always the alternative of using Ashigarus to lure fire from defending archers instead.
- Having units with upgraded armour charge in from the front take fire from defending archers and engaging defending melee units while units with upgraded melee attack/charge bonus scale from the side and charging attacking will greatly reduce overall casualties. If no blacksmith are available yet, the Encamp can be upgraded to give +2 Armour or +3 Melee Attack or +5 Charge Bonus(can be stacked with blacksmith bonuses). Naginata Samurai(mentioned in previous point) and Sword Samurai have higher base armour than other units.
This is a large factor in siege battles as 20 mins battles will require a lot more running of troops compared to a 60 min one. Infinite time battles will be even more different due the relaxed timing letting attackers take the castle level by level.
Army setup - Melee focused, equal amount of sword and spear/naginata units. Sword to kill the while spears to protect against defending cavalry and capturing towers. Avoid using siege weapons - takes too much time to shoot before attack, shooting while attacking will result in friendly fire.
Unit placement - Split the units into groups of 4-6 each group facing a tactical entry point, Gates/Low walls. Place General(if any) to a side where he can potentially boost most number of units without entering the castle.
During attack, if the defender have units near the gate, charge 2 units through in different direction to prevent the defender from gaining a flanking bonus against you, while the remaining scale from walls and attack the archers and flank the defenders. If there are no defending units near the gate, charge the units through one after another so that each unit can completely enter the castle quickly and move towards an objective, either capturing a tower or helping a defended side attack in.
During combat, use spear units as holding force while sword units move to the flank and charge in. Charge! Do not just move the sword units to the flank and order an attack. Run them a distance to the side and double click a defending unit to order a charge. This is especially important for No-Dachi Samurai as they rely on large charge bonus to deal damage. If need be the spear units can be orders to spread out wider to hold multiple defending units in place for the sword to charge. This tactic is due to Sword units usually having more melee attack while spear having more melee defence and resistance to cavalry.
Rush one unit to capture the Tenshu(Victory point), the defenders will be force to draw back to attack that unit, allowing more units to enter the castle without as much resistance. Though that one unit might not survive that siege. If possible let the unit be spear as spears are better at holding positions.
This tactic can also be used in infinite time battles though there are better one after.
Army setup - Between 8-12 units of monk archers. Remaining being Spear units to protect against defending cavalry. If playing Chokosabe, there is a unique Day Samurai that can replace Monk Archers which is better at melee combat.
Placement - Two groups of equal amount of Archers and Melee units along the longer sides of the castle - Walls with most space for defending archers.
The defender will have archers on the wall against your units. Move the Monk archers just nice into range to shoot the defending archers on the wall, they will not be able to return fire - unless they are also Monk Archers, then reposition to another section of the wall and kill off normal archers first.
If the defender also field Monk Archers, take them down unit by unit. This can be done by placing your units at an angle from the wall and attack from their flank. This will minimise casualties - also can be done when using normal Archers against normal Archers.
Once the defender run out of Archers move your Archers closer and fire in upon the defending forces till you run out of ammunition.
From this point on fight like the melee tactics with Archers being sword units(though less reliable but there is also less defenders left).
Army setup - equal Sword and Spears, 4 units of Archers(optional to open gate)
Placement - Split units into as many groups as there are tactical entry points, Gates and low walls. Archers to be near a single gate with most units entering.
Start by moving all units right before the defenders can shoot. Once in place, start attacking from sides with no or less archers. If gates are open use them. Otherwise just scale up the wall or have Archers destroy the gate with fire arrows while melee are charging.
Once the defender realised the attacking forces, some if not all Archers will reposition to defend against the current threat. Charge all remaining units as well. Archers can be placed closer to wall and shoot valuable units such as their General or the Samurai Retainers which are very effective against infantry.
Spread spear units out wide and engage defending units while Sword units cut down Archers and Ashigarus, the lost of number will cause units on the outer levels to route back into the center. Keep a least one spear unit on each General to prevent them from damaging your Sword units.
Once their Archers and Ashigarus have routed into the top level/died flank and charge the remaining melee defenders. This is also a safe time to bring in your general to conclude the combat with less casualties.
Honestly not a good idea unless you are playing Takaeda, cavalry are more expensive per soldier compared to infantry and part of the cavalry have to dismount to scale the wall and fight the Spears. But not impossible, therefore I'll provide a viable way to siege with cavalry.
Army setup - Melee focused, Cavalry being small in numbers, their Archers are not reliable in a siege. If Takaeda make sure there are at least 2 Takaeda Fire Cavalry. Bring Kisho Ninja units if possible
Placement - Split into 2-3 even groups, each facing a Gate.
Similar to melee tactics, move units right before defending Archers firing range, begin attack from least defended side. Charge infantry first, with Ninjas approaching from blind spot of Archers, use their active skill to further increase their survivability before enter castle.
Move cavalry nearer to gate, charge half(estimate, if defending forces heavily defend gate area, dismount more since the mounted will not be able to charge in until later into the fight) of the cavalry and dismount them under the wall, do not worry about their horses it is not needed for this fight and they will be regained for the next.
Scale infantry, Ninjas and dismounted up the wall, Ninja goes for least defended capture be it gate or tower. Infantry(if any) spread out and hold defending melee in place. Dismounted cavalry are still powerful melee just without their charge bonus, therefore flank with them.
Aim to open the gates if not already sabotaged before battle.
Once the bulk of their force is fixed in place and gates open, charge remaining cavalry in Unit by Unit. Use Spear Cavalry to charge down their General while sword cavalry aim for Archers and Sword Infantry. Use dismounted to kill their Spear infantry and if need be dismount more units to fight Spear, alternatively the cavalry can charge their spear from behind but retreat to charge again or at another target. That will minimise casualties from their Spears.
The above tactics mostly requires some expensive/late game units to work, therefore I separately write about using Ashigarus to siege. For early game or Oda supporters.
Army setup - As many Samurais as possible, melee to range ratio either 2:1 or 5:3. Unless defending side has weak range then the more Archers you have the merrier.
Placement - Similar to melee, equal groups near tactical entry points
Ashigaru relies on number to win, therefore getting all the number into the castle is key to victory. You should have at least one unit of Yari Samurai by default therefore use that as key against Defending General and your General against the Samurai Retainer Unit.
Start luring defending Archers to fire on your Archers. Charge melee and scale the wall. Once melee engage defending Archers, shoot defending units not engaged in combat(minimise friendly fire). Move General in to keep morale up(very important as Ashigarus route easy). Once Archers run out of ammo, scale from undefended sections of the wall and flank their units.
Make sure your Yari Samurai enters the castle after the first wave of Ashigarus so that they will be free to chase the General. In the absence of a defending general, Charge the Samurai Retainer and hold them in place. Leaving the Retainers alone will result in large casualties to Ashigarus.
Once their melee are locked in battle, bring the General in and kill the Samurai Retainer unit, followed by any Samurai units - fight dismounted once there are only spear defenders left.
Rally. while it is important to rally retreating units, it is even more important during Ashigaru battles, you might find the first unit of attacking Ashigaru routing before your Archers are even done shooting. Keeping as many units in play as possible early on will increase the advantage later - the lesser units you have the lesser defenders you can hold in place, therefore less chance to flank them.
- Sabotage gate with ninja in campaign map
- Use Monk/Missionary to boost morale
- Siege from Spring to Autumn to attrit defenders
- Fire Arrows can open gates from far
- Ninja units are good at scaling and capturing, weak against cavalry
- Units upgraded with Armour takes arrow better
- Scale lower walls to reduce casualties lost to the wall
- Monk Archers can shoot without being shot at
- Flank and rally - like you would any battle
I even skipped many points in TLDR so please take time to read through for full info.