Each of the puzzles below gives a two-word answer. Solve the puzzles and arrange the resulting 16 words into four groups of four, each with a common theme. The resulting group names form a clue for the final answer.
The four groups are:
London: Fire, Marathon, Olympics, Underground
Tower: Cell, Devils, Ivory, Water
Guard: Coast, National, Old, Security - thanks @arbitrahj for this one
Duty: Cycle, Double, Heavy, Line
The answers spell out
London Tower Guard Duty, which seems to be hinting at the Yeomen Warders, whose duty it is to guard the Tower of London.
Aggregated Answers from the Puzzles:
Written every which way
A manager's dilemma
Keeping you safe
Coordinated efforts -
Contours of a clandestine nature
Between two walls
Pulling a fast one
In excess I'm lethal
Some initial guesses:
Life Line, Cycle, Double, Cell.
Guard Old, National, Security, Coast
Medal(metal) Heavy, Devils, underground, olympics
- ??? Kinda falls apart
red white and blue?? Fire Water, Ivory, marathon
rings fire, olympics, underground, cell
Water to life
Heavy, devils, ivory, marathon which is still ?????
Possible Groupings [1,2,3,4 contradict with each other, just listing some possible combinations]
Lines/Contours: Coast, Line, Marathon, ?
Sports [unlikely]: Olympics, Marathon, ?, ?
Metal: Devils (devils metal: silver), Heavy (heavy metal), Underground?, ?
Loops: Cycle, Olympics (5 cicles on flag), Underground (System map looks like a loop), ?