First, one does not "build" a Druid's Grove. One creates it. Nurtures it. Encourages and allows it to grow. The only "buildings" that should be there are, possibly, a simple cottage built into an ancient hollowed tree or a halfling-type mound home.
Second, how much acreage/mileage are we talking? How large is the territory which you now (are supposed to) keep?
- A circle of standing stones might be nice if it has a purpose. Marking a burial area. Serving as ritual space when other druids visit for the holidays. Tracking/following astrologic phenomena (stones to mark the beginning/end of summer and winter, etc...). It should have a purpose if it is there at all. Maybe, around the edges of the Yew clearing, to lend/mark it as a particularly special place (and provide defense if necessary. by stone shaping the monoliths to form a solid wall around the dryad tree).
- You mention 3 ponds. Are they connected? I'm imagining a healthy system of streams and creeks crisscrossing the territory so that water from one pond (at the highest elevation in the wood) that sheds into two others at lower elevation. Maybe there's a nice waterfall somewhere along the way...one of the pools is where the falls come in. I would house a nice family of giant otters and their playmates/friends, the nixies, in the largest/deepest pond. Between them they should be able to keep an eye/warn if anything goes wrong with any of the waterways in the forest.
Behind the falls might make a nice abode for your druid [or druidic helpers]. If you wanted to go super Tolkien, you could make it a treant's home. Or where the nixies or some other fey resident hide treasure or make a home. Go full on 80's cartoon D&D and make behind the falls a large community of fairy dragons' lair.
- I would certainly have at least 1 treant in the area to help tend to the trees and plants. Keep an eye on various sections of the forest. Maybe, depending on the size, no more than 3.
- The sprite-filled "king's tree" needs special attention. A series of Wall of Thorns spells [without being obvious about blocking something] should be suitable to make the terrain surrounding it prohibitive to outsiders without interfering with the sprites' coming and going.
- The "Pool of Clouds": so named for its mirror-like clarity of the sky and clouds that float over head. Derithor, the pegasus stallion of the winged herd that pastures in the mountain fields above your wood, likes to water himself and his herd at the highest altitude/largest/most secluded pond. He is happy to chat if you happen to be around when he visits and has agreed to keep an eye out for strange or evil unnatural goings on in the abutting highlands that might require your attention.
-The Golden Grotto: A small grotto at the southern edges of the wood lures intruders to it with its sweet aromas. The sides of grotto are coated in the lovely bright sun-colored flowers of a large yellow musk creeper. It is handy for defending a small area in the south wood, but does require some attention to keep pruned/not let it overrun the forest/grow beyond the grotto.
-Move in some forest gnomes (if there isn't a clan already in place...or brownies or halflings or whatever small foresty folk you want to use/are suitable for your location/game setting). They'll be great for watching and defending at least some of the forest (and beneath it). Now, in keeping with their nature, you may have to go to them to get any news. But if things get too serious, they will probably let you know/seek your help.
-If you are of a level to warrant "lesser druids/acolyte followers" that don't require your direct oversight. Then sure, maybe 3 or 4 lower level druids to live there, tend the dryad's tree, learn/train their skills on the forest, assist in ritual preparation and attending to the stone circle (if there is one) and minor issues of the wood...and alert you somehow (sned animal messengers, speaking through the scrying pool, whispers through the trees, etc...) if your presence is warranted...assuming you would not be staying there always/indefinitely.
Regional Effects:
- You are aware when Necrotic energy is used within/effects your territory. [you sense the presence of "the dark side"]. This includes the presence of undead creatures.
- You know, instantly, when a non-elemental or non-fae extraplaner being enters your domain.
- You are able to heal plant life anywhere within your domain by touching a plant anywhere else you are within the territory.
- You understand and can communicate with all normal animal life, as if under the effects of a Speak with Animals spell, at all times within your territory. This power will be auto-negated if you ever charm, dominate or otherwise harm any animal life [natural/"normal" beasts, birds, reptiles, aquatic life/fish] within your territory.
- One of the pools -maybe the nearest to the Yew Tree- is your "scrying pool" and can show you anywhere within the territory or [more importantly] views along its borders.
Lair Actions: at whatever rate the MM has for lair actions, e.g. Once per round, until your next turn, whatever the terminology is.
- You are able to purify water for the entire region by touching/casting Purify Water at any point in the watersystem, within your domain.
- You are able to perform enhanced Cure Disease/Restoration/Remove Curse all-in-one ritual which works on physical as well as magical diseases/curses including lycanthropy, mummy rot, Ability loss from undead, etc... This ability may only be performed once per long rest and effects as many creatures, anywhere within your territory, as you have levels.
- As a bonus action, you are able to Dim Door through trees, as the Dryad ability, anywhere within your territory. The distance traveled can not exceed the dryad ability unless/except to travel to the Dryad's Yew tree, which you can get to [not from] any distance in your territory. This ability is facilitated by the dryad's presence as much as your own power. Harm/death of the dryad or removing her from the territory will negate this ability. Thankfully, only other druid's would know that.
- As a bonus action, within your territory, you effect vegetation within your line of sight as if under the effects of an Entangle spell, causing trees to grapple foes, grass/vines/roots to entwine, and/or trees, thickets, any plant growth to generally create difficult terrain.
Yeah. I think that works for me. I could see many games worth of interesting scenarios and adventures spawning out of this.
As might be apparent, imnsho, your role as a druidic keeper of a "grove" is to be vigilant and present and there to assist as needed. Otherwise, "let nature take its course" and the forest and its denizens proceed as they will. The forest doesn't need "fortifying" unless there's an actual threat, in which case rallying, leading, and securing the safety of the animal, fae, and humanoid residents would indeed be your duty, if you could not handle a problem yourself [even if that means evacuating/fleeing or seeking the aid of "outsider/non-forest folk"].
