Are mob lvls capped when you use the well? If I use a low tier 1 map with well, I can still throw lvl 30-40 items in it?

Yes you can do that.

even with td's?

Maps don't have td's. Most ppl use low tier well maps to find the well quickly. After you found it you can throw any white item in it.

one last note, using a low tier map sometimes ends with the last tile being the well tile, however the final map clear chest will over-ride the well and you end up not getting one.
It's a 1 in 25 for a tier 1, so not too common but it does happen

this happens to me once! I was so mad because I felt the map and its stats had lied to me xD I was also pissed because I really needed a well lol


but yes, the lvl of the cata, map or yourself does not impact on the lvl of the monster (afaik)... only the lvl of the white item you are throwing in