Thanks for the recomendations. I've been trying out Cursed Treasure. It's not bad. I must say I much prefer the diagonal view (Kindom Rush and Keeper of the Grove) to the vertical view in Tower Defense games.
The main problem I find with most, such as Cursed Treasure, is that there is no real reason to use one tower or combination instead of another, or at least that reason isn't explained in any tutorial.
I gave up on Crush the Castle TD after several levels because it seemed you could win all levels only massing balista towers. I guess I prefer the limmited tower placement system too over the free field system.
Long Way looks interesting. I'll try that next.
And Bloom Defender. I'd forgotten about that! Love it.
oh sorry for the double post, but if you prefer the limited tower placement, you might like Protector series by Undefined. That one seems to have a high difficulty, or can anyway. (at least the first one, doesn't have speed up functions though)
Another vote for Gemcraft here. Labyrinth is the best, but I still had fun with 1 and 0. The premium upgrade for Labyrinth looks well worth the price, IMHO.