Foothold

In Foothold, you start with almost nothing: a headquarters, one power plant, and ten workers. Assign them across metal, crystal and uranium deposits, spend what they bring back on power, research facilities and defenses, and hold the perimeter as waves of alien machines escalate from lone tanks to mechs, jets, stealth spider bots and mobile artillery with a mini-boss every fifth wave.

Power is the constraint that makes it a puzzle rather than a shooting gallery: towers draw from the same grid as your research, and a deficit shuts them down mid-assault. Survive long enough and you stop defending; unlock mech-warriors, jets and tanks, and march on the enemy bases yourself. Destroying every one of them is how you win.