1. This is basic to pretty much all RTS but i'll mention it anyway:

• early game you should focus on economy as much as possible and get as little defense as you can get away with
  
• once you are in the endgame (4-2 rounds left) you shouldn't get any economic upgrade anymore and just focus on upgrading defenses

That means if you can get away with only relying on your character to defeat the first waves, do it.
Towers or even a squad of melee/ranged defenders are sometimes necessary early on so you pretty much need to check what works for you.
(The defensive units become very valuable on bigger maps where attacks come from a lot of directions)


2. From what I've seen some enemies (especially the ones charging directly for your castle) can be directed into certain routes by walling only specific portions of the map and leaving others open - you can abuse that behaviour


3. Your character can grab the attention of enemies by attacking them, but you will lose it if you walk too far away - you can use that to kite them around in a circle - preferably in front of some towers or ranged defenders - If you do it correctly you should be able to do it without ever getting hit
(can also be used to split a group of enemies off into other defenses that would otherwise be out of range)


4. If your mobile defenders are outnumbered you should have them hold position (hold ctrl) behind walls or other obstacles, especially if your choice of perks/weapons/etc. prevents them from spawning again - this is much more important for ranged units than it is for melee units


That's most of the general stuff, there is much more to it since there are very specific things you can do differently depending on which perks/weapon you use - which is pretty much the charme of this game

1. On Frostsee it is very good to have the mine in the west before wave 2 (requires the architect perk) this will enable you to grow your economy much faster - you can focus on building the fishing income after that (you may want to consider not upgrading the ones you cannot consistenly keep alive, because boats need to build up and it takes some time)


I don't think there's any other specific tips that I think are usefull at the moment - there are very strong weapon/perk combinations but it's better to figure those out on your own