I am still playing GemCraft: Chasing Shadows. There are a lot of levels, and when you start going past wave 100 every time, a level or two is an evening’s gaming.
What with more than 100 levels, the map is large and sprawling. There are, however, bottlenecks through which you must pass. At some point, your next level must be X. There are probably more of these than I realized; if you happen to go the way that is required, you probably did not notice it was required, whereas crashing into the wall in the other direction sends you back this way.
There are several points at which unlocking the next hex requires a particular skill. The game is nice enough to very explicitly say that you need skill Y from field Z1, and it will not let you start a level you cannot complete. I should note that you really cannot complete some maps without particular skills, not that it’s just really hard, although that comes from the arbitrary difficulty of the wizard towers. You must unlock the tower by using up some resources, either tower attacks or spells, effectively a handicap on the level.
These chokepoints also tend to serve as story units. There is a tale of The Forgotten going through GemCraft, and you follow it across the map. It is not the world’s strongest story, but you can retrace the ruins of previous battles fought, which gives the game a bit of mystique and an aura of doom. When you look up from your shiny gems that are blowing up monsters by the dozen.