(Preface: your DM may hate you for doing this.)
Spell description:
Mold Earth - Cantrip, 1 Action, Range 30ft (affects a 5ft cube), Somatic, Transmutation. Druid, Wizard, Sorcerer.
You choose a portion of dirt or stone that you can see within range and that fits within a 5-foot cube. You manipulate it in one of the following ways:
- If you target an area of loose earth, you can instantaneously excavate it, move it along the ground, and deposit it up to 5 feet away. This movement doesn’t have enough force to cause damage.
- You cause shapes, colors, or both to appear on the dirt or stone, spelling out words, creating images, or shaping patterns. The changes last for 1 hour.
- If the dirt or stone you target is on the ground, you cause it to become difficult terrain. Alternatively, you can cause the ground to become normal terrain if it is already difficult terrain. This change lasts for 1 hour.
If you cast this spell multiple times, you can have no more than two of its non-instantaneous effects active at a time, and you can dismiss such an effect as an action.
The Tactic:
this uses the first option for the effect which is the only instantaneous effect you can spam. For all Examples I highly suggest securing a Rope or Ladder on the back wall for easy access for the Defenders. using Rope or ladder wont cost you extra movement. Just be sure to pull it up behind you, so your enemy cant use it.
See. Example A of the Diagram; Move a piece of Earth (S1-Green) to one side of your trench, this will be YOUR defensive wall. The resulting wall will be 5ft high, allowing for either 3/4 or Half cover (+5 or +2 bonus to AC respectively) at your DM's discretion to anyone behind it. Like wise anyone in the trench will receive the same benefit. The enemy (Red) will be fully exposed to melee/ranged attacks from Defenders in the trench, and ranged attacks from Defenders manning the defensive wall (S1-Green). Any creature Without a Climbing Speed attempting to Scale the 10ft back wall of the trench will need to expend Twice the Amount of Movement (20ft) to climb up. Meaning Red will use 30ft of movement to travel laterally 15ft, ending up on top of the Green block. (DM discretion: Add a Athletics check to climb the Back Wall)
Example B: Same idea as A. but just wider for whatever reason you might need a 10ft wide trench (for example Ball Bearings). For this, You move S1-Green as an Ex. A, then move S2-Blue to the front of your trench. This gives defenders inside the trench Full Cover, but also gives Attackers cover in the form of S2-Blue. However, Attacker Red Cannot reach the other side of the trench in one turn, without using the DASH action. If they Dash they cannot attack that turn, only use a bonus action.
Example C: Verdun. Follow the steps to make Examples A and B, then take a cube from the bottom of the trench (S3-Yellow) and place it where S2-Blue used to be. The Defender in the trench now has Full Cover, if you have a rope or ladder, the Defender can climb up to the Firing Stand on top of S3-Yellow to defend against any attackers, Use a Reach weapon to target Attackers attempting to hide behind S2-Blue. Get overwhelmed, jump down into the deepest area of trench, Since its 10ft deep only weapons with Reach or ranged weapons will be able target creatures at different heights inside the trench. At this point there is now way Attacker Red will be able to scale the back wall without a climb speed, or the use of a rope or ladder.
Note: Fireball will still hit you anywhere in this trench.
The Madness:
To Make Example A. requires 1 use of the Mold Earth Cantrip, so 1 round or 6 seconds.
Example B: 2 rounds, 12 secs
Example C: 3 rounds, 18 secs
Obviously, you probably dont want to make a trench mid combat, the most sensible option to use mid combat would be Ex. A. You'd make this during any downtime you may have as a quick defensive trench line to protect your camp for the night, protect a village your staying at, reinforce an area your defending long term.
How quick is it to make a trench line using Example A? (FOR 1 PERSON USING THE CANTRIP)
(remember there are 60 secs in 1 min, 3600 secs in 1 hour)
Example: 50ft trench line: 50ft/5ft= 10 rounds or 1 minute.
100ft trench Line: 100/5 = 20 rounds or 2 minutes
1000ft trench line: 1000/5 = 200rounds (1,200secs) or 20 minutes
1 mile: 5,280ft: 5,280/5 = 1,056 rounds (6,336secs) or 105.6 minutes/ 1hr 45mins and 36secs
to make Either Example B or C trenches just double or Triple the time respectively.
1 mile of B trench = 3:31:12 hrs (3hr 31min 12sec)
1 mile of C trench = 5:16:48 hrs (5hr 16min 48 sec)
I have no idea WHY in the world anyone would need to make 1 mile of 15ft deep 20ft wide trench line in a DnD campaign, but 1 Madlad of a Wizard can do it in under 5.5 hrs.
Design the top down layout of that trench however you want to impede sightlines, or allow easier access for you and your allies. Oil, caltrops, ball bearings, bear traps, or any spell capable of causing difficult terrain are all added benefits to increasing the trenches defensive and impeding capabilities. If your character has proficiency with carpenter's tools look into setting up Abatis using felled and sharpened trees, or Cheval de Frise all along the trench line.