Shellblast is kind of like a cross between Sudoku (where columns have to tally) and Minesweeper. Each bomb is a puzzle with randomly placed "pistons" which must be properly identified in order to diffuse the thing. Though the game is played on a grid, each puzzle uses a different "shape" for its bomb. Though there's no real difference between a tile which isn't used in the puzzle and one which is used in the puzzle but doesn't contain a piston, the unique shapes for each board create a nice feeling of variety as you play from puzzle to puzzle.
There are two ways to interact with a bomb: marking tiles with your cursor or moving your sweeper around. The sweeper is made up of a vertical and horizontal line which span the entire distance between two borders. In any given placement on the board, the sweeper tells you how many pistons the two glowing lines are crossing. If there are no pistons crossed, you may safely mark each of those tiles as "empty." You can also mark a bomb as a "maybe," but I didn't have much use for the maybes (every tile is a maybe until you mark it one way or another). Puzzles are solved by logically eliminating as many empty spaces as possible, and they usually have a certain "order" which they can be solved in. For example, if a row contains only one piston and you've already found it, you know that every other tile in that row must be empty.
A timer means you don't have too much time to think--take too long and the whole thing will blow up on you. Because each puzzle is generated randomly (within the "shape" of the bomb), you really do have to start over from scratch. There were a couple of things that bugged me, though. There's some storyline about needing to defeat terrorists which I actually found kind of insulting--if you're gameplay is as polished as this game's, there's really no need to bang the patriotic drum that hard.
The biggest annoyance, though, was the controls. The timer means you've got to think quick, but spending time marking the tiles is a bit awkward and time-consuming--I often felt like I was running low on time more because of the controls than because I was "thinking slowly." The sweepers are moved around with the d-pad, but there's a mouselike cursor attached to the left thumbstick which must be used to place flags. This means you'll mostly use the d-pad while thinking about where the bombs are before switching to the thumbstick to mark them. The thumbstick cursor isn't locked to the grid, which made that particular part of the interaction feel like it was ported over from a PC game (I later looked it up, and Shellblast is indeed a port from the PC). A better interface would have been to simply let me place markers at the intersection of my two sweepers--which is actually what I kept trying to do (and mistakenly placing them wherever I'd happened to leave the cursor last time I was using the thumbstick).
I didn't devote a ton of time to Shellblast's other game-modes, because what I played of the main campaign was enough to convince me that this is a pretty sweet game. 400 points is steep for the Indie Games channel, but except for some minor control quirks you won't find a more "complete" game from top to bottom.