Interesting concept--from reading your description, I gather that the goal of the game is to find the key and make it to the door. And enemies and objects also occupy the tiles on their own layer (so they are placed independently of the pipe layer).
As all of this information could only be known from reading your description, I found it was difficult to know what the goal of the game was from just playing the game, so it was a bit discouraging playing it the first time. So I might suggest making your game UI more informative by having a more guided on-boarding experience. As a specific example, for the line-up of pipes, at first I didn't even know it was a line-up, and thought all pipes had equal significance. Maybe if you allowed the pipe icons to animate and move along a path, that would show the player that once you use a pipe segment, more pipe segments will come in (maybe even show a fifth pipe segment that is unselectable).
Secondly, it's awesome that you're making a hybrid of two different genres. Because the main game layout is designed as a pipe mania type of game, I think it's a good idea to introduce the dungeon crawling component gradually, both in a gameplay sense (e.g. introduce one type of object in the first level, then more in later levels) and also in the project timeline (e.g. in your first build, introduce one dungeon crawling element like the key and the door, then more elements in future builds).
Anyway, nice job so far. Looking forward to more updates!
