Base Class: Fighter
While some fighters swing a blade with uncanny finesse or strength, and others channel the arcane or divine powers, some turned their skill set instead to the quick building of fortifications in combat. These combat engineers are skilled at battlefield repair and construction: turning a bare field into a fortress in a matter of minutes.
By Hammer or Chisel
At 3rd level, you choose to gain proficiency in either Smith’s or Mason’s Tools, and you gain double your proficiency bonus for any check made with them.
Quick Build
At 3rd level, when you gain this feature, you learn two Quick Build options of your choice. Once per turn, you can use your action to use one of your quick build options. You have two uses of this ability, and you regain all expended uses of it when you finish a short or long rest. You gain an additional Quick Build option of your choice when you reach certain levels in this class: 7th, 10th, 15th, and 18th level.
Master Fixer
At 7th level, you learn the mending cantrip and can cast it without material components a number of times per long rest equal to your intelligence modifier. The casting time becomes one bonus action, and it can fix a break or tear up to 5 feet in any direction (as opposed to 1 foot as written in the spell).
Tank Warfare
Starting at tenth level, you gain proficiency with land vehicles, if you do not already possess it. Any land vehicle you are commanding or riding in has its Hit Points Doubled and its AC increased by 5.
Ready Builder
Starting at 15th level, your magical construction is available whenever battle starts. If you roll initiative and have no uses of Quick Build remaining, you regain one use of it.
Fortress Builder
At 18th level, you can expend both uses of Quick Build to create an instant fortress (as the item) that lasts for 8 hours.
The fortress is a square tower, 20 feet on a side and 30 feet high, with arrow slits on all sides and a battlement atop it. Its interior is divided into two floors, with a ladder running along one wall to connect them. The ladder ends at a trapdoor leading to the roof. When activated, the tower has a small door on the side facing you. The door opens only at your command, which you can speak as a bonus action. It is immune to the Knock spell and similar magic, such as that of a Chime of Opening.
Each creature in the area where the fortress appears must make a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage on a failed save, or half as much damage on a successful one. In either case, the creature is pushed to an unoccupied space outside but next to the fortress. Objects in the area that aren’t being worn or carried take this damage and are pushed automatically.
The tower is made of adamantine, and its magic prevents it from being tipped over. The roof, the door, and the walls each have 100 hit points, immunity to damage from nonmagical weapons excluding siege weapons, and resistance to all other damage.
Quick Build Options
The Quick Build feature lets you choose options for it at certain levels. The options are presented here in alphabetical order. They are all magical effects, and each one is associated with one of the schools of magic.
If an option requires a saving throw, your Quick Build save DC equals 8 + your proficiency bonus + your Intelligence modifier.
Empty Pit
You create a 10ft by 10ft hole in the ground no more than 20ft deep within 5ft of you where no space is currently occupied. Alternatively, you can create a 40ft by 5 ft trench no more than 5 ft deep with at least one square within 5ft of you where no space is currently occupied.
Hunting Blind
You create a 5ft by 5ft structure. Anyone within this structure gains advantage on Stealth checks. This structure has an AC of 5 and 5 HP, and requires a Perception of 20 to detect as present.
Loot Drop
You can choose up to 5 of the following items to appear in a small pack at your side:
(20) crossbow bolts, (20) arrows, (10) sling bullets, (2) rations, (1 gallon) water, (1) antidote, (1) grappling hook, 50ft. Of hempen rope, caltrops, ball bearings, (5) torches, (1) tent, (2) bedrolls, (1) block and tackle
Powered Turret
You set up a self-firing, crossbow turret that faces any direction within 5ft of you. This turret fires once per round, at the start of your turn, in the direction it faces. This turret has an AC of 5 and 12 Hit Points. Ranged Weapon Attack: +5 to hit, range 80/320ft., one target. Hit (1d8) piercing damage. This lasts for 10 rounds or until dismantled or destroyed.
Raft
You can create a wooden raft, 5ft by 10ft. This raft has a movement speed of 20ft and can hold 250lbs. This raft has an AC of 5 and 5 Hit Points
Rapid Repair
You can repair a structure or item within 5ft of you for (1d10 + your fighter level) Hit Points.
Siege Engine
You create a small catapult or trebuchet within 5ft of you. This is considered a medium siege weapon and can be manned by one person. It has an AC of 10 and 15 Hitpoints. Rocks launched from the siege do 3d10 points of bludgeoning damage. This weapon takes an action to reload and an action to change the aim. This uses the attack bonus of the wielder. This considered a martial ranged weapon.
Stone Wall
You set up a small stone barricade within 5ft of you. This wall is 6ft tall, 10ft wide and 2.5 ft. thick. It has an AC of 17 and 25 Hit Points. This wall has 2 arrow slits within its wall.
Watchtower
You can create a wooden tower 30ft tall and 10ft square. A staircase leads to the exposed top. This tower does not have a railing. It has an AC of 10 and 25 Hit Points.