"By Dawn's Early Light" had a somewhat weak alibi for the killer because Rumford was one of few people with a key to access and alter the charge of the cannon.

Another person with access is the so-called
"Officer of the Day" but Columbo doesn't
bother to ask who that is. This always
puzzled me. Also, Columbo asks Rumford
if there is anyone out of the past who
might have a motive to kill him and
Rumford stupidly says there isn't anyone.
He could have diverted Columbo's attention
by making up someone.
BTW-there is a clear allusion to homosexuality
in this sequence. Columbo asks if maybe
there was jealousy over a women and Rumford
EMPHATICALLY says no--added to the fact that
he is surrounded by so many young boys.