Most TDs are all about co-op. It's what makes them so much fun -- at least for me. But most can be beat solo play, even though they state best for x amount of players. The good maps will automatically close down a lane if no player is present in that spot.
Here's the only two I've played/beat solo -- they're OLD:
Marvel vs XMen:
This was a really fun TD for one or 2 players, but beyond that, it was too unbalanced.
(You have to build at the very top, right in front of the exit portal to beat this one.)
CounterStrike:
Better with a group of friends, but still enjoyable single play.
Burbenog can be single play, if you're a master of the game.

It's REALLY freaking hard and requires lots of running around and micromanagement.
With BattleNet, which is free, there was never really a shortage of friends to games with.
<]=)