Relevant links: complete version, enhanced edition
First off: do not confuse this with Dungeon Defenders, with an s. That is a 3D multiplayer game with some vaguely tower defense styled trappings, and which has gotten a lot of press. This is a straight tower defense game, done in flash, which never got any press.
The shtick is that you’re a villain in your lair with a bunch of pesky “heroes” coming to get you, so you set up defenses and traps to stop them. You lair is underground, so you can also dig tunnels and find treasure and precious ore.
It’s not the most polished game, and graphically it’s a little simple, but there are a few things I still like about this one: the digging mechanic is great. It allows for pathing manipulation and a means of earning bonus resources and a way to diversify the level design, all in a single mechanic.
This is also the first tower defense game that I know of which implemented melee units for stalling enemies in a particular spot. I’d never claim that it was the first, I play a lot of tower defense games but my knowledge is by no means complete. Never the less, I give this one credit for implementing that mechanic three years before Kingdom Rush popularized it.
Finally, there’s the protagonist unit which functions much like a hero unit in, again, Kingdom Rush. That’s fine and all, but what I really like here is the fact that the protagonist gets equipment (i.e.: loot) over the course of a level. This can really add something when you’re focused on defense but also running a side campaign to reach that tantalizing treasure chest juuust a little out of reach.
There are two versions of this. The original version is here, and is complete and fun. There’s also a Dungeon Defender Enhanced Edition with some extra features, along with some paywalled content. Which you decide to play is mostly about how tolerant you are of microtransactions, though it’s worth pointing out that the Enhanced Edition also has online saving. I find that to be pretty valuable with flash games.