2 Answers
2

Some more hints for defense grid that I used to achieve the gold badge in all levels:

Upgraded gun turrets are very underrated. One of the first things I like to do in most maps is to find the necessary choke point and place several gun turrets in that position and get them up to red as fast as possible.

Find key strategic positions on the map. Find and establish 'kill zones' where you know the enemy is forced to move through at least twice (once on the way to the core and once on the way out). If you can shape the enemy's path, do it in a way to force them to move through your 'kill zone' multiple times.

Do not use up all your money immediately. The more money you have (as long as your cores are intact), the more interest you gain per tick. This is key if you are aiming for the high scores for gold badges.

Don't forget to upgrade your anti-air turrets on maps with fliers. On most maps, a single well placed red turret will be sufficient to defend against all the waves of air, though some of the more difficult maps will leak a few through but your guns and or cannons can mop those up.

On maps where you cannot shape their path, there are still almost always places where the aliens will move through twice, and you want to prioritize the upgrades of turrets in those areas first.

Split up alien paths if possible on maps with multiple spawn points. If all the aliens group up together, your turrets will have a harder time to handle all of it (unless you go with heavy splash towers like inferno and meteor).

Some towers, like temporal and command, cost the same for each upgrade level, so are good candidates to max first. Almost all towers have their range increased when upgraded, which is always good to have.

Interesting question: generally either strategy is effective. If you build very few buildings, and upgrade them to their maximums, you will do well. Likewise, if you build a great many buildings and never upgrade them, you'll also do well (except in very small levels, and you need to make sure to path the aliens well).

My #1 upgrade criteria is coverage. How many aliens will this building be able to hit, if I upgrade it? You always want to get the most bang for your buck, so if you've got a building on a corner where the aliens have to traverse its entire firing arc, upgrade it. But if you have a building where the aliens only have to pass it once, leave it low.

The towers I always upgrade are Meteor Towers and Temporal Towers. Meteor towers are extremely effective, and their absurd range means one tower can usually cover the entire map. Temporal towers I upgrade because they're cheap for their effectiveness. If you upgrade your towers, it makes a lot of sense to invest in temporal towers.

The thing to remember about not upgrading, is that it allows you to put down many more towers, and, on large maps, this is critical. You want to shape the paths of the aliens to keep them on the map for as long as possible. This cannot be done if you upgrade your towers constantly...You won't have enough money.

Unless you're going for an achievement, you'll want a balance. You'll want to use low level towers to shape the path of the aliens so that they stay in range of your upgraded towers for as long as possible. If you can't shape their path, use temporal towers to slow them down, so as to keep them in range of the upgraded towers for as long as possible.