There are multiple obstacles in Braid. At various times these can be either helpful or harmful, but in any case they are often a great factor to gameplay.
“Clouds are "bridges" in which to get to areas otherwise inaccessible. They are fired by Cannons...Clouds
“Cannons are a prevalent obstacle in Braid. They tend to shoot either Monstars, Mimics fireballs...Cannons
“Spikes are an obstacle in Braid. If Tim touches a spike, then he "dies" (see Time Manipulation...Spikes