the duc is :
(defrule
	(players-building-type-count target-player watch-tower > 0)
	(unit-type-count militiaman-line >= 4)
	=>
	(up-reset-search 1 1 1 1)
	(up-reset-filters)
	(set-strategic-number sn-focus-player-number 0)
	(up-find-player enemy find-closest math-goal)
	(up-modify-sn sn-focus-player-number g:= math-goal)
	(up-find-remote c: watch-tower c: 1)
	(up-set-target-object search-remote c: 0)
	(up-filter-include cmdid-military -1 -1 -1)
	(up-find-local c: infantry-class c: 10)
	(up-target-objects 0 action-default -1 stand-ground))

it workes with a goal called tower-demolition-goal which is set to "1" when tower is detected
Then it is set to 2 when militians are trained + men at arms UP

(defrule
	(current-age == feudal-age)
	(players-building-type-count target-player watch-tower > 0)
	(goal tower-demolition-goal 1)
	(unit-type-count militiaman-line > 4)
	=>
	(set-goal tower-demolition-goal 2)
	(chat-local-to-self "Tower demolition project Phase 2 !")
	(chat-local-to-self "DEMOLITION"))

I want it to be set to zero when the tower is destroyed
=>
(defrule
	(current-age == feudal-age)
	(players-building-type-count target-player watch-tower == 0)
	=>
	(set-goal tower-demolition-goal 0))