This game is clone of 3DO Arcomage, but have much uglier art quality. One good innovation is multiplayer, but it's very bad working and always disconnect.
Easy gold trick - make party of elf snipers, goblins, sylphs, sky dragon and 2 more ranged units. Then go to hard/epic goblin madness challenge, set up arrow tower and go sleep. On morning it was 1.700.000 gold and mission success :)
This is the most unbalanced defense game i've ever played. Most major issues: 1) unable to set attack priority (last, weakest, etc.). Thus only way to make focused fire on the enemy tank - by building bunch on towers in one spot ant pray the towers can kill 1st tank in row fast, because otherwise they will just mindlessly damage each enemy tanks without killing them (fire is spreaded insteaded of focusing) 2) towers effeciency totally not match tower cost, especially comparing basic tower with other towers.
On other hand all this makes game more challenging, BUT - it makes me think i fight with game bugs, not with tanks.
To make quickly all levels on "glowing frame" level, max pure gems mastery + high grade starter gems, and restart level till you have enough gems to build one lvl 8 red gem, then just spam wave bricks, red's splash effect will tear them all up. On level without reds, use high-lvl lime, they shoot like chaingun on lvl 8-9.