The possible answers are
FREEZE and its homophones.
Said the cop to the crook who was making a haul:
Policemen say "Freeze!" to criminals.
"You seem like a pattern that's seen on a wall.
A "frieze" pattern is a pattern that is equivalent to itself by translation and/or rotation; basically, a wallpaper pattern.
When a jailer has pity and unlocks your gate,
A jailer frees you when he unlocks your gate.
You may well rejoice in the change of your state."
Freezing is a change of state from liquid to solid.
It may start in a flower, end towers with bells
A freesia is a type of flower, and bell towers are called "belfries".
And is in the reversal a seer foretells.
Reverse "a seer foretells" and you can see (and hear!) "FREES".