Theres two ways to do this that i think is most effective
First off heres the defences of a lvl 10 NPC:
2200 Rolling logs
1375 Defensive Trebuchets or Rockfalls
I think theres about 5k towers but id just sent an attack to the NPC i was getting the information from so i dont really know
So, its obvious not to send Cataphracts and Cavalry because they'll just get eaten up by the Abatis. If you have tryed to take a lvl 10 NPC in the past you will know that the main trouble is the Warriors, so this first technique is quite usefull for getting rid of them.
Send around 50k archers with high level archery and military tradition. send about 50k warriors to take out most other things in lots of waves and hey presto, the defences are down and other waves after the 2nd catch the city with no defences, the citites yours!
Ballista glitch, this is the expensive way to do it but cheapest in the long run as less losses are inflicted. Send loads of ballista with again, high level millitary tradition and archery, i dont know how many ballistae are needed but id take a guess at around 50k. Which means 2 attacks from each city can be sent at a time so less ballistas will be needed as less defences will be there, i would risk a guess it would be a bit like this:
1st time: 50k ballistae
2nd time: 45k ballistae
3rd time: 35k ballistae
4th time: 25k ballistae
5th time: 10k ballistae
and so on and so forth.
REMEMBER TO SEND TRANSPORTERS!
Hope this helped anyone and everyone looking to capture a Level 10 NPC!