Private Mining Company

TL;DR you've put a great twist on a saturated genre, has the potential to be popular but the level of polish, fidelity and QoL will be compared with other games in the genre as a bar to entry.

Positives

• I like the twist of persistent units, a sort of blend of XCOM meets TowerDefence
• I like the idea of mercenaries being weaker but somewhat expendable
• I liked being able to pull in waves early

Negatives

• The health bars underlapped the units, making it hard to keep track sometimes
• I couldn't figure out how to cancel placing a mercenary

Opportunities for improvement

• Indicator of number of waves left (Unless you're going for a more gritty/helldivers feel)
• Splats or impact effects for creatures taking damage or dying
• Range indicator circles when placing or hovering over units
• Unit Distinction
  • Summary of who died/survived - maybe even give them names or badges for duration of service
  • Different coloured hats for in-level identification
• Menus and UIs felt a little bland, you could consider leveraging a UI asset pack such as this
• Consider adding in a incoming wave indicator as seen in games like BubbleTanksTowerDefence, which would work nicely with your speed up feature 
• While a lot of the features are staples of the genre, I didn't know there was a mining drill - perhaps something to highlight