Have a name and basic description for all the heroes, towers & monsters written up in the game wiki. No specific stats are needed yet except for what items it takes to build if the thing in question is a tower or monster.

Example Wiki Write-Ups:

[Tower] Curse Tower
Items: CA
Applies a curse, dealing DoT damage to all heroes within it's range as well as causing 50% more damage from all sources for afflicted heroes. Has the smallest range of any tower.

[Monster] Scrapyard Robot
Items: T
A robot thrown together from pieces of junk you found laying around the lab floor. This robot is a quintessential of a basic lab defense mechanism. While not the fastest or most deadly thing around, it doesn't have any major areas of weakness (at least as compared to other devices of its quality).

[Hero] The Samurai
A Samurai from ancient imperial Japan. Raised by an ancient oath to protect the modern world, it retains its deadly skill in combat. Heavily armored and very, very dangerous in close range combat.

Have at least one image for each monster, tower, hero, projectile, and item. There is no need for complete sprite sheets for animation yet, but these will need to be created in the future.

Have generic sound effects for each any unit (i.e. heroes and monsters) being hit, kill, attacking, etc. plus generic sound effects for towers firing a projectile and those projectiles exploding and/or hitting something. These sound effects should not yet be specific to different towers/monsters/heroes/etc but general effect that all of these things share (e.g. all heroes and monsters will, for now, make the same sound when attacking). Also have one main background theme song.