Tower Defence Games

There are many of them out there. One of the more difficult and also well polished one is Xeno Tactic (I still don't understand how to play xeno tactic 2, but 1 is fairly easy with a good learning curve). It also has been around for a long time.

It is best to play it here.

I've manage to get quite far (level 86) in the last mission, and I think I can beat it soon.
Here are a few tips I've found.

Maze: a good maze is crucial to the game, especially to avoid being overwhelmed in the very beginning as mission 6 crawlers start with quite a high health. Also, do not build any sam turrets or walls in the beginning, as you need the money for vulcans.

You will also need to have a double-ended maze to make the aliens go around in circles (selling 1 wall to open 1 end, and then block the other end with 1 wall). A good thing to do is to group old aliens with new ones especially when you get to 50's and it is especially important to keep bosses in the middle of the group, or else your sams will abandon the big blob and shoot only at the boss.

SAM Turrets: some people underestimate the full potential of these as they only do 256 damage at max, and a measly 8 damage unupgraded. However, no other turret (not even plasma or sonic) can match its damage when a group of 20+ monsters are crammed up in little space. You should have 2-3 when you get to level 50, and add more to keep income coming (4 to get to 70, and add a 5th one before 90).

DCA's: the whole game is about anti-air. Ground units will die eventually using the blocking method, but airs will just soar right through all your defences, and that is a huge limitation in the game. Maximize your air defence and save your lives until level 100 (you must lose a few lives in 93 and a lot in 100).

Plasma Turrets: when fully upgraded (BFG Tower), they are excellent boss killers. Get 1 BFG before level 32, and another one before level 48. That will take care of all bosses before level 60 in a fairly short time. They are also quite good against air. However, don't get more than 2, because, after the 60's, you will have like 40 bunched up, and plasma is no use against ground at that point.

Freeze turrets: they are crucial on the later levels where air units are just almost unbeatable (begin around level 40). Build them around DCA's and fully upgrade them to zero tower, or else they are no use. They can also freeze ground units, saving money on turning them.

Guide to DCA (and Zero Tower) building:
Level 7: 1 DCA (accompanied by 1 or 2 volcans in case of lopsided defence)
Level 14: 2 DCA2
Level 21: 2 DCA4 or 1 DCA5+1 DCA2
Level 28: 2 DCA5
Level 35: 1 THW + 1 DCA5 + 1 DCA3
Level 42: 2 THW + 1 DCA5
Level 49: (this is 2 air-boss) 3 THW + 2 Zero Tower
Level 51: (the 7-level pattern restarts) 4 THW + 2 Zero Tower
Level 58: (this is 2 air-boss) 5 THW + 3 Zero Tower
Level 65: 8 THW + 3 Zero Tower
Level 72: 11 THW + 5 Zero Tower
Level 79: 14 THW + 5 Zero Tower
Level 86: 17 THW + 6 Zero Tower

You should kill every single air unit up to this point and not lose any groundies, after level 88, sell all your SAM Turrets (you should have 4 or 5) and buy 3 more THW

Level 93: 20 THW + 6 Zero Tower (can lose 2-4 if 1 less)
Level 100: (ideally) 37 THW + 14 Zero Tower (you will lose 12+ here, with only 20 THW)

After all air passed, sell all DCA and buy 6 SAM Tower and 6 BFG.
When only the 2-million health slug boss is left, change everything to BFG.

PS: don't spend an unhealthy amount of time playing this game... just saying