Thanks for the advice, atomiccat. I shall focus on my turrets and health.
Any thoughts on what class I should choose? (My understanding is that there are 4 classes with different abilities.)
Squire and mage are best for turrets/baricades and easiest to play Squire gets barricades that block and damage enemies as they come close and Harpoon and cannon ball turrets and mage gets magic missle, fireball, Lighting and a Slow high damage turret. The AOE turrets( fireball and lighting/ bouncing and slicing blockade are the best just place them On or around Choke points and Defend and heal them and you should do fine.
The Monk and huntress are harder to play, monks place auras and huntress places traps, they don't block enemies so you need some good damage or someone else placing Blockades in along with traps/auras or turrets for max efficiency And they take a lot of maintenance, Because they take damage as they are used against enemy instead of them being directly damage by the enemy.
Imo the best upgrades are
Squire is best tanking and having turrets that have high health and high damage
Mage, High turret damage,and aoe and get a staff that shoots more then 1 projectile otherwise its basically useless, unless its being used to buff your turrets
Huntress needs a mix of aoe , health and damage for traps and some damage and speed for attacks
Monk needs a mix of everything you can go pure dps/health and use auras as a minor support or a mix or auras and dps which is what i do since i tend to play a support role healing and placing auras in choke areas to hinder and hurt enemies, and Strength drain aura is basically useless unless you are going dps and placing it right in area u are fighting in
and for all a little casting speed always helps unless you are going dps without turrets