Tucked away in a corner of the realm where prairies yield to foothills, a wizard's tower stands amidst a circular hedge-maze.
This maze is apparently well-maintained: green and perfectly manicured, about 50 m in diameter, and based on concentric circles, around a tower which stands five stories, about 15 m tall. There are four entrances to the maze, aligned to the four cardinal points. The hedge itself is about 2 m high at the outside edge, rising to about double that near the tower: climbing the hedge wall would not give away any information about possible first-floor doors in the tower.
Careful viewing of the tower reveals not windows but four floors' worth of arrow-slits, each surrounded by carved, presumably defensive runes that glow disturbingly in the morning sun: trying to fly to the tower would likely be dangerous.
It will be necessary to traverse the maze or to hack through it. The civilized way will take several hours: there are four independent blocks, one inside each entrance, so a simple strategy like "keep left" will not work, but the maze is definitely solvable.
Trying to hack through the maze, on the other hand, is much more dangerous: the maze will resist such attempts in an escalating fashion, with a variety of plant-based effects.
At the perimeter, the show of force is basically symbolic: slender tendrils offer little resistance to travel, seed pods explode at random like popguns, flowers emit pollen clouds that cause teary eyes and brief coughing fits. If the attacks persist, the hedge ups the ante: stouter branches attack, the clouds of pollen produce severe choking fits, bole-like growths suddenly appear and release hallucinogenic gasses. Eventually thick limbs armed with razor-sharp thorns and sharp spikes strike with unerring aim, dense pollen clouds cause blindness and life-threatening asthma attacks, and exploding pods launch hard seeds like crossbow bolts. By the time a party cuts half-way through the maze it will probably be dead.
If the party uses elemental attacks like fire or cold, the hedge ecology is prepared: burning it will release clouds of noxious gasses in several grades of deadliness, while those trying to freeze it will find that it has a large base of anit-freeze producing material to fight off this form of attack. Water attacks will only make it healthier unless they are overwhelmingly powerful, and air attacks will just rustle the branches and leaves, again unless hurricane-force winds are produced. The hedge will still fight back with its natural weapons as long as it is able. Nature-oriented characters like druids, barbarians, or rangers should get a bonus on checks to realize the hedge's capacities in this regard.
If, however, the attackers decide to retreat, the hedge will continue its attack for a brief interval, until it decides that the retreat is real and not just a strategem (exiting along the entrance cut so as to avoid further damage would be a good way to convince the hedge of one's good intentions...)
Once the attackers have left the maze, it begins to repair itself, from the interior to the outer wall. And it will remember the attackers for the rest of the day, until nightfall: whether they try to hack their way in or enter peaceably by one of the four entrances, the maze will respond at the same level of resistance at which it left off.
The next morning, the maze will have "forgiven" them and be willing to let them enter to solve it, or meet them with the initial mild reaction if they are so rash as to choose to hack their way in again. Obviously the hedge goes dormant at night, thus allowing it to forgive and forget. It might be thought easier to try the maze at night: the wizard thought differently and graved those runes around the arrow-slits. Depending on the time of day and the damage already done, what comes to defend the maze and tower might be anything from a murder of ravens to a tribe of night-gaunts or other demons.
Obviously the wizard of this tower is an Elf-friend of no mean stature, with access to powerful elven magic and the special secret plants that elves use to defend their own forests. Repeated attacks on the maze might incur the wrath of the elf Goddess Herself. Such enmity would likely manifest as a curse of barrenness, not just on the party, but also on the regions where they travel. Eventually someone will trace the pattern of crop failures and bad harvests back to the party involved. At that point there would need to be *serious* penance done to appease the anger of the Goddess visited on a large part of the local continent...
And what does the tower contain, that it is guarded so well? (Perhaps it is the maze that is the prize: traversing it in just the right way might grant a plant-based power, anything from Talk to Plants to Summon Ent...)