Here's an example: (I've highlighted the line that you need to insert)
Code:
}
        wooden_castle castle requires factions { northern_european, mesoamerican, middle_eastern, eastern_european, greek, southern_european, } 
        {
            convert_to 0
            capability
            {
                recruit_pool "Mailed Knights"  1   0.2   2  0  requires factions { england, scotland, france, hre, spain, portugal, milan, venice, papal_states, sicily, Normans, } 
                recruit_pool "Viking Raiders"  1   0.2   2  0  requires factions { denmark, } 
                recruit_pool "Polish Nobles"  1   0.2   2  0  requires factions { poland, } 
                recruit_pool "Boyar Sons"  1   0.2   2  0  requires factions { russia, } 
                recruit_pool "Hungarian Nobles"  1   0.2   2  0  requires factions { hungary, } 
                recruit_pool "Byzantine Cavalry"  1   0.2   2  0  requires factions { byzantium, } 
                recruit_pool "Arab Cavalry"  1   0.2   2  0  requires factions { moors, } 
                recruit_pool "Mamluk Archers"  1   0.2   2  0  requires factions { egypt, } 
                recruit_pool "Sipahis"  1   0.2   2  0  requires factions { turks, } 
                recruit_pool "Mongol Light Lancers"  1   0.2   2  0  requires factions { mongols, } 
                recruit_pool "Turkomans"  1   0.2   2  0  requires factions { timurids, } 
                recruit_pool "English Huscarls"  1   0.2   2  0  requires factions { Saxons, } 
                recruit_pool "Hobilars"  1   0.4   3  0  requires factions { england, } 
                recruit_pool "Border Horse"  1   0.4   3  0  requires factions { scotland, } 
                recruit_pool "Mounted Sergeants"  1   0.4   3  0  requires factions { france, hre, milan, venice, papal_states, sicily, Normans, } 
                recruit_pool "Scouts"  1   0.4   3  0  requires factions { denmark, } 
                recruit_pool "Jinetes"  1   0.4   3  0  requires factions { spain, portugal, } 
                recruit_pool "Polish Shooters"  1   0.4   3  0  requires factions { poland, } 
                recruit_pool "Kazaks"  1   0.4   3  0  requires factions { russia, } 
                recruit_pool "Magyar Cavalry"  1   0.4   3  0  requires factions { hungary, } 
                recruit_pool "Skythikon"  1   0.4   3  0  requires factions { byzantium, } 
                recruit_pool "Desert Cavalry"  1   0.4   3  0  requires factions { moors, } 
                recruit_pool "Arab Cavalry"  1   0.4   3  0  requires factions { egypt, } 
                recruit_pool "Turkomans"  1   0.4   3  0  requires factions { turks, } 
                recruit_pool "Mongol Horse Archers"  1   0.4   3  0  requires factions { mongols, } 
                recruit_pool "Turkish Horse Archers"  1   0.4   3  0  requires factions { timurids, } 
                recruit_pool "Theigns"  1   0.4   3  0  requires factions { Saxons, } 
                recruit_pool "Dismounted Polish Nobles"  1   0.2   2  0  requires factions { poland, } 
                recruit_pool "Peasants"  0   0.000001   0.999  0  requires factions { england, france, hre, denmark, spain, portugal, milan, venice, papal_states, Normans, Saxons, } 
                recruit_pool "Highland Rabble"  0   0.000001   0.999  0  requires factions { scotland, } 
                recruit_pool "Southern Peasants"  0   0.000001   0.999  0  requires factions { sicily, byzantium, } 
                recruit_pool "EE Peasants"  0   0.000001   0.999  0  requires factions { poland, russia, hungary, } 
                recruit_pool "ME Peasants"  0   0.000001   0.999  0  requires factions { moors, egypt, turks, mongols, timurids, } 
                wall_level 1
                gate_strength 1
                tower_level 1
                free_upkeep bonus 2
                law_bonus bonus 2
                recruitment_slots 2
            }
            material wooden
            construction  2 
            cost  1200 
            settlement_min village
            upgrades
            {
                castle
            }
The subsequent castle levels miss that line. It would be logical to increase the number of slots for the higher levels accordingly.