Observe from the tower (4)
I believe the answer is:
keep
I believe this is a double definition.
'observe' is the first definition.
('keep' can be a synonym of 'observe')
'the tower' is the second definition.
(thesaurus)
'from' acts as a link.
(Other definitions for keep that I've seen before include "strong position" , "Save" , "Conserve" , "Observe - reserve - preserve" , "Retain; part of castle" .)