It's not going to blow anyone's mind, but Old Monterrey really is sort of the platonic ideal of a mobile platformer. You can move left and right. You can double-jump. And you have to traverse challenging levels to find and rescue a bunch of goats. Sometimes it's just that simple!
What surprised me is how Old Monterrey expands from its humble tutorial levels. After the first five or so levels, which are basically old-school Mario-style straight shots from left to right, the levels get much bigger. Each level becomes its own little Metroid-esque exploration challenge.
Thankfully, the game provides a very useful map. Also each goat you rescue operates as a checkpoint, making deaths not too terribly annoying.
In total, Old Monterrey has just under twenty levels that increase in challenge as you go. It won't take more than a couple hours to blast through the whole game, but then again, it's totally free. Hard to blame a game for not overstaying its welcome when that's the price tag attached!