Lord Bucket’s list is nice. Let’s keep it even simpler:
Step 1: Put down 1 tower in a nice choke point or where path crosses / returns.
Step 2: put the highest dual level gem you can afford in this tower. As LB notes, Purple/Green early on, Red/Yellow later.
Step 3: place a few walls to choke off paths or force wind arounds where possible and still cheap.
In the early maps, that’s really all there is to it. Don’t get distracted and spend mana around on multiple towers or gems.
Add some skill points to each of the top row (mana stream, fusion, masonry). Like 3 each.
Now on each board after initial set up above, eventually add 1-2 traps at start of path with a level 2-3 purple and/or green gem.
Along here you may need to add a second tower with a lower dual gem while waiting to upgrade your main. Like when your main is level 4 or 5 and the mana cost jump is just too big. But rarely go with 3 gem towers (map specific).
once you unlock beam, bolt, barrage, add 3 points to each of those and every now and then add at least 3 more to the original 3.
focus on armor traps at beginning of maze routes and yellow gems in tower(s) and black where available.
upgrade freeze to at least 6. and put points in true colors and wake of eternity when available.
you should not need to bother repeating boards for points, nor hunting down achievements. many achievements will just happen in course of normal game play. but you might want to review the building achievements to hit right at the end of a board just for fun during late phase.
talisman fragments are not terribly great, but add them in of course as you get them. upgrade the damage ones as you can.
I never used amps but others seem to like them.
play a few of the vision fields (like 5 or 6) to get the extra xp and skill points. some are actually not that hard.
Finished the game with following skills:
True Colors, Mana Stream, Fusion at 15
Masonry and Freeze at 12
Crit Hit at 6
Resonance, Beam, Bolt, Barrage, WoE, Armor Tearing, Traps at 3
I was at level 112 when I started finishing map.
Remember: build a tower, put a dual gem in it, place a few walls, win.
The only differences map to map are where you may want to increase Masonry or Freeze depending on maze (if you need a lot of walls) or condition (if you need to freeze a lot).
Hope this helps