You don't need 700 troops to take a town. In fact, I managed to take 2 towns and 3 castles with a troop limit between 120 and 130 using only my own troops. The key to this is:
1. Have only elite troops while attacking fortresses. Hopefully you and your companions have high training skills - with them, you can turn a large bunch of peasants into elite troops just by training in one week.
2. Do not try to kill all defenders in one battle - wear them down instead. Start the siege with 100 Rhodok Sharpshooters, order them to hold their position immediately, and see how they kill enemies in much larger numbers than they suffer losses themself. Having a good medic with high surgery helps too, so many of your losses will be only temporary.
Once your troops stop firing due to the lack of ammunition, retreat from the battle and start to build new ladders/a siegetower directly afterwards. After several battles, the garrison is defeated.
Should your archers be inefficient as the enemy has only infantry left with strong shields, so that they barely kill someone, retreat to your nearest castle/city, swap your archers with powerful melee fighters like Huscarls and start an assault in the next siege attempt.
Obviously, this works much better if you only have to build ladders as you don't have to wait as long as with siege towers.
