Seems a little OP.
Pros:
+Sticks to walls
+Upgrade adds 2nd head

Cons:
-Only up to level 2
-20% firing speed
-10% sentry health

Nice idea

Yea that seems pretty cool. please continue brainstorming stats for it, it would be much appreciated.

Pros
+sticks to walls
+faster turn speed (so it can aim down)
Cons
-only upgrade to lvl 2
-10% smaller sentry range

Pros:
+Sticks to surfaces

Cons:
-immovable.

引用自 Zectric：

Pros:
+Sticks to surfaces

Cons:
-immovable.

I agree with these stats. Simple but fair, and still different and usable in multiple situations. The only downside is you have to be sure the sentry is in a spot you want it to be before you put it down and upgrade it, instead of keeping it at the spawn and upgrading it then moving it up to the frontlines.

Life changes on levels level 1 100hp level 2 150hp level 3 200 hp
Attack changes on levels level 1 5 damage each shot level 2 5 damage each shot as its a minigun turret level 3 20 damage plus 5 on splash damage
costs 200 metal
can only be used i you have the rescue ranger(becuase engie is not spiderman he can not climb the wall)


Pros:
Sticks to sufaces
3 upgrades level 1 average turret level 2 minigun turret level 3 a missile turret

Cons:
Immovable
Can not shoot behind or up

hope you like my ideas

Pros :
Sticks to surfaces
20% faster firing speed
10% higher magazine
Can fire in every direction
Fires at incoming projectiles, depending on distance when the projectile explodes (Optional)
Can build up to 3 Sentries (Optional)

Cons :
Cannot be upgraded
Immovable
50% more damage recieved from any type of attack
200 metal used to build it
Makes a louder sentry sound
Can't be used with the Wrangler

I think what we need to bear in mind here is balance. Let's come up with ideas to not make it OP, but not render it useless. I would see this in the same family as the mini-sentry: No upgrades, no reparing, just refill ammo, and a bit smaller than the regular sentry.
I suggest the following:
+Can be built on walls and ceilling
+Fires in every direction
-50% damage voun.
-15% range.
-15% slower firing speed.

Yeah I think that Pros:

+ Sticks to surface
+ Smaller than standard sentry (lvl 1 is mini-sentry sized and the hit box is only that bit)

Cons:
- immovable
- Only upgrades to lvl 2 (wich would be smaller than a lvl 2 regular, but probably larger than a regular lvl 1 - this increases the hit box)

and is then attached to a Cc weapon like the mini-sentry is (depriving the engineer of other usefull wrenches) is pretty balanced, considering that putting it in harder to reach places like the ceiling make it harder to upgrade and repair, but also harder to reach to sap and that the fact that it will no longer be a physical obstruction (which does matter- trying to sneak past a sentry nest as a cloaked spy becomes a bit easier) if it's on the ceiling.

Edit: Actually faster turn speed would be useful so maybe also:
+ faster turn speed

- 10% less sentry health

perhaps if it had lower firing rate and had slwer react time? then it could be upgraded?

give it a different top than the normal sentry head.

stats:
+sticks to walls ( not ceilings)
+on level three, it fires a slowish projectile that does 150 damage in a AoE
-projectile can be reflected
-takes critical hits
-10% slower fire rate

Well, how about this?

+sticks to any surface with sufficient space
+can be placed at an extended range
+can be repaired and upgraded at an extended range

-can be sapped at an extended range
-cannot build teleporter
-can only upgrade to level 2

+Sticks to any surface.
+Can be placed at extened range
+Can be upgraded/repairded at extened range.

-Does not inform you when building is taking damage
-Falls off wall or ceiling if Sentry health drops below 35%

cloaks at level 2