what should i add to an isenguard army to give it a good chance of sieging a castle
any tactics for sieging

also are there any troups you would reccomend to defend a siege, and also any useful defensive tactics

thanks in advance

A ballista, some detonation bombs, bezerkers, isengard trolls (to push siege towers), ladders and battering ram.

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"I am the Flying Spagetti Monster. Thou shall have no other monsters before me"
-FSM.

To do a siege you will need:

Ladders
Siege Tower (with pushing Troll)
Demolition Teams
Battering Ram
Some Orc Warriors to make up the numbers so you don't get thinned by bows

For defence is would say:
Shield Armed Warriors, both Orc and Uruk for resilience
Bows or Crossbows
A strong leader to put behind the gate with some shield warriors to speed bump the enemy and bottleneck the entrance (trolls are good for this)
Spear armed Orcs or Pike armed Uruks to help support the gate defence force
Ferals / Berzerkers but hide them away until the enemy is over the walls and then unleash them

There are other ways but these are the setups i would go for.

Defence tactics, well block up the gate with infantry, have high defence or shield armed troops on the wall and something nasty hidden away to rush those that break through the wall and into your castle / camp.

'FireKnife'

ok great. thanks for that!! by the way:

can you get a troll to climb a siege tower?

how do you cover your siege bombs when your enemy is firing from above them, it means the person in the fortress can get line of sight on anything.

oh and it is really difficult to get enough number when i am spending loads of points on siege kit. i am making a 1000 point army and already after i put in 2 bombs and a balista my points left have plummeted.

how usefull are the upgrades you can get for a siege ballista? or would i be better off getting a few more warriors?