In the final version of the game we gave the players 30 seconds to complete each stage as a compromise between allowing them enough time to figure out what to do and to execute their plan.
Using the timer we decided to spawn a bonus points giving fruit pick-up ( inspired by Pac man ) every 10 seconds, giving a maximum of 2 pick ups per level. This tied in nicely with other games spawning a reward the player could grab, at the risk of leaving any safe zones of the screen they might have found. It would have been more ideal to have this tied to the player performing an action, such as having glooped all the enemies on screen.
An element that went late into the game was spiked platforms. Previously the player could freely jump up and down platforms, easily avoiding enemies. Spikes were added to create more threatening platform sections and channel the player through certain parts of the map. They also make the player feel more uncomfortable when jumping over gaps ( as failure now costs a life ) and add a sense of danger to the screens.
Invincible enemies were added around the same time as spikes to create moving obstacles for the player to avoid. These, unlike the spikes, would require the player to time their actions to avoid colliding with them. Originally these enemies were going to be a particular colour, but it was difficult to communicate their invulnerability to the player as we were using a range of colours for the enemies already. A neat solution was to cycle the colour used on invincible enemies. That way they stood out easily when the player glances at the screen, and separated them from other enemies.