To begin with, You spawn in a testificate village, which you are given the role of protecting from invasions. The monsters come through a portal in a stronghold, which you will have to destroy, but you have to do this along with protecting the villagers at the same time.
-You start off with a wooden pickaze, a clock, leather armor, iron sword, 128 redstone dusts, 64 raw wood, 320 arrows, a piston, 5 dispensers, 10 seeds, 20 pork chops, and 10 pressure plates in a village chest.
-The village is located in the middle of a connection of caves that you can explore.
-If all of the villagers die, you lose. If you destroy the portals before all of the villagers die, you win.
-You are given 3 minecraft weeks to prepare materials for the defense, and build any fortification around the village, before the invasion begins. You can gather materials during or after invasions, where there will be breaks during day time before they come again. Creepers, spiders, and endermans come randomly during the day, but not often.
-The invasion starts at night, so you have day time to prepare the defenses.
-Monsters spawn randomly in the overworld and all head towards the village you are defending. The amount of mobs spawning depends on the amount of portals left. You have to find their source portals and destroy them to stop mobs from spawning and to win the game.
-The portals are made of obsidian, so you need a diamond pickaxe to destroy it.
-To find the portal, you need to build a nether portal yourself. You will go into the nether to kill Blazes and get their Blaze Powder. Then you combine it with ender pearl to make eye of ender. This is to find the last portal. It is guarded by an ender dragon and 5 hostile endermans. You don't have to enter the ender portal, you just have to destroy it, but first you need ot deal with the boss. The ender dragon won't fly away, it will stay there and fight until it dies.
-While in the nether, there is a time limit. The time limit is a day and night for each of the remaing villagers you have left. If you have 5 villagers left, you can spend 5 days and 5 nights in the nether. You must keep track of time with a clock, which will still keep track of time in the overworld. The clock is given to you when you begin the game.
-The assaults last for infinite waves until you destroy the portal.
-After each night, more mobs are added little by little.
-If you die, you will go back to your spawn point or your bed. If the mercenaries die, they disappear and drop their equipments, which you can give to another mercenary.
-Ballistas tower: You craft a ballista, which you can use right away, but you can craft an attachment and combine it with the ballista. This allows you to attach the ballista to blocks. The attack speed of ballistas are increased, as compared to when you fire the ballista without being attached to a block.
Ballista towers uses arrows, which you have to craft and replenish when it runs out. There are 3 shooting modes: You can shoot 1 arrow at a time, 2 arrows at a time, or 3 arrows at a time. The con of 3 arrows at a time is that you use more arrows, obviously. But it is crucial to spray 3 arrows a time if there are a huge number of mobs.
-Hirable testificate mercenaries: Requires gold ingots to hire. They will fight to their deaths, and you can heal them by feeding them cooked food. They act like tamed animals once hired. You can right click them to position them at different places. The melee mercenaries will run to attack any mob that comes within 6 blocks of them, and the ranged mercenaries will attack depending on the range of given weapon. Afterwards, they return to the position you perscribed to them, rinsing and repeating. They will still fight even if unhired, but they won't listen to you or follow you, so you can't place them liek you place cats or dogs, with the adde benefit of attacking nearby mobs.
If you place mercenaries next to a ballista, they will sometimes fire the ballista at incoming mobs.
To beat tower defense, you are going to NEED mercenaries. It will be extremely difficult to find and destroy the portals by yourself. The portal rooms are guarded by powerful mobs, such as Blazes and ender dragons, as well as some ghasts. In addition, if you can't kill the boss before night, more endermans will spawn.
I suggested this hirable mercenaries in another suggestion, but this is very applicable here. The link is in my signature.
Wolves and cats: Cats can be stationed to deter the movement of creepers, or you can have it run freely and chase creepers, deterring creepers from its path. Wolves will do the usual of attacking mobs that are attacking the player.
-Block platforms: These are regular block platforms of dirt, cobblestone, etc, which you can build tall enough, so you can position your hired mercenary archers to protect them from zombies and creepers, because archer mercenaries have lowest health of the mercenary types.
You can litter the map with these platforms in a way to make a maze. Mobs only try to attack mercenaries or the player when these mobs get attacked by the mercenary or player, and the mobs that don't get attacked head for the village to kill the villagers.
If you block ANY route to the village with blocks, such that mobs can't jump over or walk through, 5 ghasts, more endermans and creepers will spawn the next night to deal with your blocking.
-The House: Houses "heal" the number of villagers. Villagers reproduce, the more houses you have. It is crucial you rebuild the houses lost in the invasions, or else you will have less village lives left. You can increase the amount of maximum villagers by building more houses, which spawns more villagers, which represents your life.
-Traps: You can craft all sorts of traps to kill the mobs or slow them down. You start off with a piston trap, 5 dispensers, 10 pressure plates, 320 arrows, and 5 spike traps in the village chest, along with some other items(such as leather armor, and iron sword, and a clock).
Water: You can use moats to slow down the movement of mobs. Or oppose mobs with flowing streams of water. Keep in mind that mobs come from random directions.
Pit fall: You can dig very deep holes that mobs might fall in, but because of the AI, they tend to walk around and empty hole, but they will walk on trap doors! So if you have a deep hole coverd with trap doors, which is activated by redstone. You have got yourself a killing machine, but does not work on ghasts and Blaze because they are flying.
Spiketrap: Requires iron. Works like redstone. You sprinkle them on the ground, and when mobs walk over it, they take damage.
Piston trap: Set up on ground, and when mobs trigger a pressure plate, it will fling the mob into the air, which creates fall damage for the mob and slows it down.
Poisoned meat: Crafted by combining either pork chop, beef, or chicken with potion of poison. Potion of poison is crafted with a cauldron, which requires you to be later into the game. Can be placed on the ground. Poisoned meat attracts all zombies nearby, and poisons the zombies within one block radius. This is helpful because zombies are the main bulk of the mobs.
Dispenser traps: You have seen these before. Basically, it is a pressure plate activated trap that shoots arrows. But the ballista still does more damage. You can have a row of dispensers, and have it shoot at mobs walking through it.
Sticky trap: Made from slime balls, which can be placed on ground like redstone dusts. It stops mobs walking on it for a short amount of time. Each goo piece can be used 25 times before disappearing.
(I will add more traps later)
Enderman: Removes your blocks from your fortification randomly, so zombies, creepers, and skeletons can pass through your walls to kill vilagers. You have daytime to repair these damages.
Creeper: Explodes near mercenaries and player, destroying blocks and causing damage. Also damages other monsters, so you can use it to do area of effect damage to nearby mobs.
Zombie: Primary invader. They come in larger numbers than any other mob.
Skeleton: Ranged units that can attack ranged mercenaries with ease.
Slime: Mainly a distraction and tank for your mercenaries or pets.
Ghasts: They only come when you block entry to the village with blocks, which otherwise will only appear near portals.
Blaze: Flying mobs that comes at the end of each week.
Zombie pigman: They are hostile when they come. They move faster than zombies and deal more damage.
Ender dragon: Appears near portals, guarded by 2 Blazes
Multiplayer: Play online with your friends, get more resources, build more houses.