Awesome game. Hard difficulty seems to be tuned very well, took quite a few attempts to win. As a prototype not much to complain about honestly; just needs more mechanics, random offset of enemy spawn locations(?) and some polish. Keep the levels short & sweet as you have; turn based games can become a slog with many units and tiles to traverse. 

Always love seeing berserker type units in games.

Like to see what units have over-watch and the effected tiles, seems to pop up out of no where. Ability to see when each enemy unit will take their turn. Pan camera with middle mouse or right mouse. (Maybe) Zoom in/out to see full extents of attacks like Monarch's missiles. 

Fix camera lerp when selecting units one after the other. Camera currently jerks around in the browser version.

(Completely Optional) Recommend hyper specialising each ally unit, to avoid making them all do similar things. Amplifying their uniqueness and character. Goblin is a good example focusing heavily in tech with no direct offence, so is Monarch if she had a min attack range for main missile and mortar -- forcing her to keep distance. For example:

• Blackbeard: swap assault rifle with a short range stun gun that disables flying units ability to fly until they start their turn. Pushes target to the end of the turn sequence for this and next round.(Takes it's turn after everyone else) Useful for keeping high value targets from attacking too soon or bypassing an enemy. While stunned, they can't react.
• Shallowtail: swap assault rifle for a mine launcher. Less damage than Explosive mine, 2d3 for example. But can be reused as much as needed. Covering the land with mines. 
• Kobold: using the terrain mimic to jump from terrain to terrain, throwing satchel charges on un-aware enemies, ground or buildings that explode when triggered dealing massive damage. Planting and detonating(all) both take 1 power each. Can't be detected even if adjacent tile is occupied as long as we don't move. Has bonus detection avoidance. Has an energy knife or shovel, dealing massive damage if from stealth. Can build walls/cover while being undetected, even in the mist of battle.
• Etc.

Add Linux version if possible.

Would love to have more player artillery & support units. The ability to spawn a wall is fairly unique, having a soft cover version would be nice. Some other "ideas"...

• Hard cover platform that during player's turn retracts to the ground and can be walked on becoming no cover. On enemy turn, move back up and become hard cover. If something is on top of it, it can't move back up.
• Spawn flying drones(3/3) that follow an ally, peppering nearby enemies. Can be targeted by enemy.
• More effective mines, beserkers tend to be the only unit that activates them. Yet they receive so little damage. Adding reliable 4 perhaps?
• High armoured(4-6) ally that focuses on holding down a 9x9 area around itself, those tiles act like water. Ranged attacks that would hit nearby allies are re-targeted to self. Little to no offensive abilities, can taunt map wide(?). Armour helps avoid ally AOE damage.
• Sniper focused ally, using a giant anti-material rifle that can pierce through hard/soft cover hitting all tiles along the way. Requires a full 2 power to reload. No other weapons, and fairly weak health wise due to favouring move speed and cloaking.
• Ally(Furnace) with massive heat capacity(20) and absorbs reactor heat from nearby allies. Ability to cool allies and recover their reactor stress levels at the cost of increasing own heat. Uses some type of heat weapon, generating more heat to self on each attack. More heat means more damage. On death or reactor failure, explode based on current heat. 3-4x the standard explosion size when at max heat.
• Acid Spitter ally, focusing on removing enemy armour over range while also shredding them at close range with corrosive claws. Every tile it moves on and around a 9x9(Only when stationary) becomes an acid tile for 2 rounds causing damage to both allies and enemies.
• Minigun styled ally, focusing on dealing with unarmoured enemies and flying as each bullet deals 0-1 damage. But can fire many bullets is a single volley. (When an attack is activated, call that attack multiple times one after another to avoid adding all damage rolls into a single value and dealing damage. Which would negate the whole point of this character.) Can suppress enemies, reducing their movement at the cost of missing most shots and increasing heat. Can shoot huge volleys, with a heavy heat cost. Slow movement.
• Mind control type ally, that can take control of 1 enemy unit at a time. That unit is AI controlled but acts as an ally unit. Enemy can attack that unit, and it attacks nearby enemies. While active: high heat cost each turn, cannot move and is vulnerable.
• Flame tank ally, that focuses on burning down forests and soft cover. Turning tiles into fire tiles that last for 2 rounds, dealing damage every round.
• Level that has a rouge AI launching missiles randomly at all units. Goal is to avoid killing enemies outright, as the more enemies that exist the less chance you will be hit. All while storming some fortress, eliminating external generators or just trying to get to some evacuation zone.

Best of luck mate, game dev can be brutal.