"Tech tree" is a clear metaphor to anyone who has spent some time playing strategy games... but if you're looking to communicate the idea to non-gamers, then maybe "ladder" is a better term? It strongly suggests independence of each technology, which is accurate at least for your first release. The interdependence of a strategy-game tech tree is probably a closer match, but you risk losing some non-gaming readers. Bonus: your terminology of "levels" carries over nicely to a ladder-based system.