So I've been working on my homebrew continent for awhile now (67pg including img/ half way?) and while working on it I had an idea for a campaign.

The Theme: Stationary base defense and town building.

Basically the party would be contracted to defend a strange portal that spits out monsters that steadily grow in strength and they have to build defenses to hold off these hordes. The main method of doing this would to be build up a nearby town by building better buildings, hiring mercenaries, getting more citizens etc and building traps, mazes or other things around the portal. Most of the campaign would take place in a fairly small area unless leaving the area for a specific thing or mission and would be a much more casual, RP focused campaign with the exception of the attacks or side missions.

Players would also level based on town progress rather than EXP but this would give them additional abilities (possibly even lair actions near the portal) as they build up the town. Things like leader ship abilities to increase NPC success against the monsters while the party is somewhere else.

Side note: I recently purchased the baby bestiary books so players could also train their own monsters to fight.

Still mostly in the concept phase but what do people think?

It seems like an interesting concept, and I would love to play a campaign like this. Two tips I can give:
-Mass combat/tower defense can be difficult. If you have mercenaries/townspeople fighting beasts while the players are somewhere else, this can get boring for the players. Either have some way to resolve this quickly, or check if the players are willing to play multiple characters.
-5e D&D has no support for this, but there are other books that can be converted/implemented. The Pathfinder "Kingmaker" Adventure Path also lets you build up a kingdom. I only know the first two modules of it, but it might contain some useful tips & tricks if you can get your hands on it. Matt Colville has also recently released a book called "Strongholds & Followers" that is quite well received.

It seems like you're trying to make a Tower Defense game with some RPG elements, which isn't really what D&D is about. You would be better off taking existing Tower Defense games that already do what you are currently describing and just adding a bit of RPG flavour to them - it will save you on having to rework the mechanics of it.

I play D&D for the vast world  get to play in, for the opportunities and variety of options it offers, to explore character concepts and so on. I play for the stealth missions, the dungeon crawls, the political intrigue, the character developments. I want to be the one facing the traps, to find new routes, work out new approaches, to solve situations either with wit, with blade or just with words. I like to roleplay a character. Being stuck in a small area to just battle monsters non-stop and defending a stronghold from them with new installations/upgrades (i.e. Tower Defense game) is pretty much against everything D&D is for me. So, with that in mind and giving that I don't like Tower Defense games at all, I'd give this a very hard pass. I'm sure there are others who'd enjoy it (they may like Tower Defense games or play D&D for different reasons than me), but you wanted our opinions and mine is this is really not for me.

EDIT:

Having re-read this I think I am coming across more negatively than I intended so let me add: please keep thinking outside the box and finding new ways to enjoy the game. While this idea is not to my personal tastes I do commend the creativity that went behind it, please do not be discouraged by my opinion above: it is never a bad thing to approach games or anythng from new perspectives or ideas. Keep at it! *thumbs up*

It could be difficult to keep things new, but it's a very interesting idea and it sounds like it could be fun.  Although if you're homebrewing a continent, a campaign based in one town seems somewhat less than ideal.

Thank you all for the comments so far :) But I think I should clarify a little bit. While the primary focus would be the area around the portal/town, you would still have to explore more of the country.

Whether it's for resources for the town, find specific NPC's to help you, additional requests by the people who hired you or to find specific items. This would require a fair amount of exploration. This is why you would need traps and soldiers, since the portal still functions while your somewhere else and you need people to defend the portal. I also wouldn't make players sit through the NPC fights, i'd just give a summery.

Honestly it'd be less Tower defense and more strategy game and mystery with finding the portal source

I've actually had an idea similar...Minus the tower defense parts.  But instead of one portal...They keep poping up around the continent, each one spawning different things, some helpful, some dangerous.   You could easily turn this into several towns, make them spread their resources out, and make them explore. 

I'm curious, if monsters keep coming out of it, why do the PCs have to protect it?

Quote from Wysperra >>

I'm curious, if monsters keep coming out of it, why do the PCs have to protect it?

 To keep the monsters from attacking nearby villages and to ensure badguys don't get access to it