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