Are you pre-placing them in descr_strat? If so make sure the co-ordinate is actually in the region the watch tower is supposed to be in. Code example from my 'Stone Forts' tutorial:
Code:
region Region_Name
farming_level 4
famine_threat 1
;fort 68 103 me_fort_a culture middle_eastern
;fort 68 103 me_fort_a culture middle_eastern
;fort 68 103 stone_fort_a culture northern_european
;fort 68 103 stone_fort_b culture northern_european
;fort 68 103 stone_fort_c culture northern_european
;fort 68 103 stone_fort_d culture northern_european
:fort 68 103 knightly_order_fort_b culture northern_european
watchtower 66 27
The watchtower model is defined in decr_cultures (example southern_european culture), make sure that custom models are properly entered and in their correct folders. I believe the last part of the entry has to do with the 'foundation' texture.
Code:
watchtower data/models_strat/residences/SE_watchtower.CAS, watchtower_roman